The word slip has several meanings:

1. To slide accidentally and lose your balance or footing. You might slip on ice, on a wet floor, or on loose gravel. When your foot slips, it slides out from under you in a way you didn't intend. A rock climber whose hand slips loses their grip on the rock face.
2. To move quietly or quickly without being noticed. A student might slip out of the classroom during a fire drill, or you might slip a note to your friend during lunch. The word suggests smooth, unnoticed movement: “She slipped away from the party early.”
3. To decline or get worse gradually. If your grades start to slip, they're dropping little by little. An athlete whose performance is slipping isn't doing as well as before. Test scores can slip, attention can slip, and even memory can slip when you're tired.
4. A small piece of paper, like a permission slip you bring home from school for your parents to sign, or a slip of paper with a phone number written on it.

The phrase let something slip means to accidentally reveal information you meant to keep secret, like when you let slip that you're planning a surprise party.