Simplified version: SSDs or more precisely NAND flash memory has some technical quirks. You can't just overwrite stuff like you do with magnetic hard drives. You have to actually delete things. Also you can usually only delete full physical blocks.

But data rarely fits neatly into physical blocks, so instantly deleting data would mean every time you delete something that partly resides in one block, you would need to save the data in that block you want to keep, wipe the whole thing then rewrite parts of it. That's a lot of unnecessary stress on the hardware and also more work in teh writing and rewriting process.

So SSDs use a similiar approach as old magnetic hard drives. Blocks aren't physical deleted but marked as no longer used. Which works well for hard drives where you can just overwrite data, but for SSDs this is also far from optimal as with every new write you want to commit you have to pre-check if this unused part of a block is already deleted and usuable or not? In the latter case you either need to do the "save some of it, wipe the block, rewrite" oparation you tried to avoid before or simply go on and try to find another actual free block to use. Which decreases performance obviously. And at some time you would have a lot of wasted half-full blocks and would need to organize your data to reclaim these

Enter TRIM... where periodically (usually when the system is idle anyway) the blocks that are marked as deleted are checked if there's still used data in it (those are then copied to a completely free block and the whole block is wiped to free it up for the next write.

In layman's terms it's basically anolog to defragmenting a hard drive in the brackground. When the drive isn't doing anything important it spends time rearranging data only partly using a physical block to wipe the whole thing and also to wipe blocks that are completely marked as deleted to keep everything neat and organized with fully used blocks and wiped blocks available for re-use.

PS: so-called continues TRIM is available also. That will basically do the whole "everytime a part in a block is marked as deleted write the other data to a new block an wipe the whole block"-routine in real-time... but after the regular writes are done to not clog up the system. But periodic TRIM is easier on the drive and usually sufficient.

PPS: TRIM is part of the drive's set of commands. So all the data handling involved doesn't need to be handled by the file system. You just issue the TRIM command to the drive and it handles everything internally.