Program signaling lets applicants mark a limited number of programs they are especially interested in, giving those programs an extra indicator when they review applications. The number of signals and which specialties use them depend on the application cycle, so check the current rules.
Signals are most useful when spent on programs where you are a realistic fit and genuinely want to train. Spending them on programs that clearly screen out your profile wastes them. See the ERAS entry for where signaling happens in the application.