to answer: to respond to what someone has said, asked, or charged
This is the basic verb for responding. Its core idea is reaction to a prompt: someone speaks, asks, or accuses, and this word names the reply that comes back. That is why it fills so much of the dialogue in Hebrew narrative, where a conversation is told as one person speaking and the next answering.
The range stretches naturally from ordinary back-and-forth talk to two more pointed settings. In court it is the response a person gives to a charge, which is how it slides into the idea of testifying or speaking up as a witness, for good or for harm. In prayer it is what God does when he hears: to answer is to act, to grant the request, not merely to reply with words. So when the Psalms beg God to answer, they are asking him to do something, not just to speak.
Across all of these the prompt-and-response shape holds. The senses below sort the contexts where the responder is a person in conversation, a witness under accusation, or the LORD answering those who call on him.