slot is simply a ledger number in Stellar implementation. Once previous ledger closed nodes start by proposing candidates for the next "slot" and then vote on it.
Some relevant info can be found here
key implementation details
The Herder considers a ledger number to be a "slot" in the SCP protocol, and transaction set hashes (along with close-time and base-fee) to be the sort of "value" that it is attempting to agree on for each "slot".
