The phrase implies active effort to concentrate on specific sounds or information. During the lecture, make sure to tune in to key points the speaker makes.
Pro Tip 2/3
Scheduled Event
Often used to remind or schedule a time to attend to a broadcast or event. Don't forget to tune in to the live webinar at 6 PM!
Pro Tip 3/3
Not Always Literal
Can be figurative; doesn't have to be about audible content only. Tune in to the body language of the audience to gauge their interest.