Alexa, like other virtual assistants, is designed to listen for a specific wake word. In Alexa's case, the default wake word is "Alexa". When this word is detected, Alexa begins recording your voice and sends this recording over the internet to Amazon's servers to be processed and responded to$.$ So$,$ in a sense, Alexa is always listening for its wake word, but it isn't recording everything you say and sending it to Amazon. It only starts recording after it hears the wake word.