That +# value acts kind of like additional item levels in how it boosts attack of weapons and defense of armor. When you Soul Match an item that already has a +# value on it with another having a greater or equal +# value, the number goes up a single digit, increasing the bonus.
So a simple +1 can just be transferred as you please using Soul Matching, but you can also Soul Match an item with +1 with another +1 to get a +2. This process can be then repeated by Soul Matching two +2 items together to get a single +3 items and on and on an on, to whatever the +# cap is in the current game. This is mostly reserved for endgame as the final means of item level progression in NG+ for the new quality type of items, Divine, but it can be poked at in NG now thanks to purple quality items having them.
One of the major factors in this is that the number simply cannot go up more than a single digit at once, so if you use a +3 as a material, the recipient item will still only go up a single digit, no matter if it was a +1, +2, +3 or even without a +# bonus altogether. Another one now is that the +# values on purple items cannot be Soul Matched into those Divine tier items, so whatever +# values you've accrued during NG have to be abandoned eventually.