What is the meaning of "Position in mempool" for a transaction?
To illustrate this let's imagine the mempool as big vertical list of transactions.

Let this list be ordered by the feerate each transaction is paying, the higher the feerate the higher the transaction in the list.

Miners will under normal circonstances take the highest virtual MB of transaction in this list/queue.

This website is a good visualizer of this concept.

NB: The tip in this context is the transaction in the mempool paying the highest feerate.