We are all insane here.

The "mempool" is made up of all bitcoin transactions that have yet to be put (aka "confirmed") in a block. They just hang around like cartoon babies in heaven, waiting for the stork to take them to their family.

The more time that passes since the transaction gets into a block, the higher the probability that it can't be double-spent. What that means practically, is that the recipient of the bitcoin should wait a certain number of blocks before they should consider their bitcoin safe to spend. This is a risk-tolerance model. Some dummies wait 0 blocks. 6 blocks is super safe. Bitcoin targets a rate of 1 block every 10 minutes.

Every tx pays a fee to the miners. The fee represents how badly you want your transaction to be in the next block that is mined. It's a "fee market".

During times of extreme bitcoin usage, the mempool can become backed up with lots and lots of transactions, and only the highest fee bidders' transactions will make it into the next block.

During times of low bitcoin usage, the mempool can fit all pending transactions in the next block, (I.e. the mempool clears) meaning the transaction fees are low and it is relatively cheap to perform on-chain transactions.