You use the phrase "at least" to talk about the smallest or lowest number of something:
There were at least 20 people waiting in line already when I got there.
You can also use "at least" to talk about time:
I try to get to the airport at least 2 hours before the flight leaves.
This means that the speaker tries to arrive at the airport 2.5 or 3 hours before the flight leaves, but certainly no later than 2 hours before the flight.