The Bitcoin memory pool includes all pending Bitcoin transactions that are yet to be confirmed.
Miners get increasingly picky when there is a major spike in the network activity, which is why users have to pay higher fees in order to get included in the next block faster. Otherwise, their transaction might get stuck or outright rejected.
According to a recent tweet by a popular mempool explorer, all transaction with a fee below 2 sats/vB will be purged:
The mempool is now full again for nodes with default limits. The minimum fee for new transactions is currently 2 sats/vB, and any TX below this fee rate will now be purged/rejected from the mempool. For high priority transactions, we currently suggest a fee rate of 144 sats/vB.