Here's the drill:
Flow, by itself, does nothing. One stack is consumed for each spell cast, with no effects. BUT, if you do have at least on stack of Flow, certain spells and artifacts will trigger effects. In the case of Rock Cycle, you can cast it infinitely as long as you have mana, since it generates 1 Flow after use, and having Flow while using it makes the spell go back to your hand.
Similarly, Frost by itself does nothing. 3 stacks disappear and cause 100 damage. But, certain artifacts trigger effects while doing Frost damage or by having Frost stack. Selyce's Weapon also affects Frost - for each stack the hit target has, it does extra 60 damage, recovers 1 Mana and erases the stacks.
The same with Trinity, it only activates effects if you have 3 stacks, and before that does nothing. The difference is that Frost needs to be stacked on enemies, while Trinity should be stacked on yourself (you still can get Frosted by certain opponents though).
Root makes the target stay immobile (though it can still attack) as long the debuff lasts. Anchor is similar, but to yourself and as long the attack animation lasts. Conversely, most spells do not Anchor you, so you can freely move while using them and they still will play out normally, which means stuff like the double claw attack can theoretically hit once two different squares.
Jams are useless, 1 Mana cost card. But for each you play, certain other cards attack more/with more power.
Consume is like Exhaust from Slay the Spire; the card is gone for this combat.
Fragile makes the target take 50% more damage for an amount of attacks equal to the amount of Fragile stacks.
Flames ignite panels and do damage to anyone standing on them, but after a few cycles of damage, burn out. A bunch of cards affect this.
Poison is also similar to StS, it does damage equal to the amount of stacks each time cycle, and then the stacks go down (by half? not sure).
Crack means a panel or more gets cracked, and if somebody walks over it, it will Break, rendering it useless until some time passes and it fixes itself.
Weapon Damage obviously only affects weapons, and thus is useless for certain loadouts (Chrono Saffron) but also extremely effective for others (Gunner). Meanwhile Spell Damage affects all your attacks but your weapons.
Defense reduces damage but only if taken directly - to your health, not to your Shields. There's an artifact that affects that.
Mana Regeneration is how fast Mana recovers, and besides certain artifacts affecting it, the more cards you have, the more mana regen you get too. You can check how the milestones by looking at your folder and seeing the "0.1 -------" lines, and if you look at the bottom, you can see your total Mana Regen. and the previous stats. You can also see Luck, that increases both difficulty and loot rarity, and can be affected by artifacts (and ascensions, I am guessing).
On the shop, Pacts are like StS curses, but with a time limit (like Chaos Boons in Hades) for the drawback. Upgraders let you give your cards one of three random buffs. You can upgrade cards multiple times but it requires multiple Upgraders. "!" rooms during runs you offer you upgraded cards as rewards.
Hostages come in 3 flavors, black clothes with a coin sack - gives you money if saved; green clothes with a chest - gives you an artifact; and the nurses, that heal you for 100 HP.
Bosses won't give XP or money unless killed, but will heal you for 400HP and randomly appear on future rooms to give you spell/weapon damage, summon turrets, attack enemies or heal you. There's both a "genocide" and a "pacifist" route with different final bosses. If you do plan to go for the latter, make sure to save Reva and try not to die - she gives you one free revive.
Red "!" are "dark world" paths for your current level. Dunno what actually changes in dark world besides the shopkeeper getting a mood, and the background changes.
You start with a free Remover, that erase a card from your deck, and can buy more at the shop, or get some from artifacts. Keep that card flow steady.
When selecting your character, press the "details" button to check what their weapon and starting artifact do, and which spells they start with. For Violette, after using her weapon, each direction is a buff; back is Shield (use before Corset), down is Mana, front is an attack (?), up is Trinity. Move to one of the markers to activate it.
Finally, the aim cursor is always 4 squares away since that's the default range of most spells. Some are 3 squares away (the step sword and the whirlwind that Saffron starts with, for instance), so you only need to aim one back.
And above all else, give it time. You are going to get used to the pace, just remember you don't have to go frenetic with it. Calm down and read the panels to know what is safe.