According to https://mempool.space/docs/faq:

What does it mean for the mempool to be "full"?
When a Bitcoin transaction is made, it is stored in a Bitcoin node's mempool before it is confirmed into a block. When the rate of incoming transactions exceeds the rate transactions are confirmed, the mempool grows in size.
The default maximum size of a Bitcoin node's mempool is 300MB, so when there are 300MB of transactions in the mempool, we say it's "full".

It explains how we got there, but doesn't elaborate on what that means for Bitcoin.

Right now we are at  354 MB / 300 MB, but if that doesn't have any effect why are we even talking about a limit in the first place?