The "minrelaytxfee" is the minimum fee rate you need to pay so your transaction broadcast. The default amount which is used by most of nodes is 1 sat/byte.

The "mempoolminfee" is dynamic and depends on size of total unconfirmed transactions. It's the minimum amount of fee rate you have to pay so your transaction can enter the mempool. The purpose of this limitation is to prevent the mempool size from exceeding a certain amount (300 MB by default)