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".
+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.
By default, Bitcoin Core allocates 300MB of memory for its mempool, so when a node's mempool grows big enough to use all 300MB of allocated memory, we say it's "full".
Once a node's mempool is using all of its allocated memory, it will start rejecting new transactions below a certain feerate threshold—so when this is the case, be extra sure to set a feerate that (at a minimum) exceeds that threshold. The current threshold feerate (and memory usage) are displayed right on Mempool's front page.