In Act II, scene 1, Lady Macbeth rings a bell when she has gotten the grooms drunk enough to pass out. This is the signal for Macbeth to come and murder Duncan.
The signal is a bell.
Please refer to Act II, Scene 1, lines 31 and 32 in which Macbeth states the following: “Go bid thy mistress, when my drink is ready, she strike upon the bell. Get thee to bed.” Skip a few lines down to lines 62 – 64. “I go, and it is done; the bell invites me. Hear it not, Duncan; for it is a knell that summons thee to heaven or to hell.”