A Multi-pool is a pool that allows to mine more than 1 coin
Well, the multi-pool have set-up 1 stratum / coin so you can point your miner to any coin that te pool allows, that's more easy for the user because you don't have to change the pool for every coin that you want to mine.
The multi-pool don't produce coins, they just mine the most profitable coin of your algorithm at the moment, then swap to btc and then buy the PoS currency that you want at market price, so then the PoS coin usually go up because there is a buy pressure every day (Not all the coins xD).
Hope that will help you