A slot leader is a node or stake pool selected to compile, verify, and record a block of transactions on the Cardano shared ledger.
The slot leaders are picked and assigned to slots before an epoch begins. Each of them is expected to be online when their particular slot is reached.
The more stake the pool controls, the greater the chance of being elected as a slot leader to produce a new block.
You may learn more about Cardano stake pool operation HERE.