When a Bitcoin transaction is transmitted to the network it first gets verified by all of the Bitcoin nodes available. After it successfully passes verification it goes and sits inside the “Mempool” (short for Memory Pool) and patiently awaits until a miner picks it up to include it in the next block. So the Mempool is basically the node’s holding area for all the pending transactions.
Here’s a short video about this (the mempool is the unconfirmed transactions pool basically):
All nodes have a different RAM capacity to store unconfirmed transactions. As a result, each node has its own rendition of the pending transactions, this explains the variety of Mempool sizes transactions counts found on different sources.
But how do the nodes avoid from crashing due to overload by the Mempool size? If the Mempool size gets too close to the RAM capacity, the node sets up a minimal fee threshold. Transactions with fees per kB lower than this threshold are immediately removed from the Mempool and only new transactions with a fee per kB large enough are not allowed access to the Mempool.