Let’s say that you slept for too long, and now you realize that you are going to be late for school. So you jump out of bed, put on some clothes, and run out the door. You make it in class just in time, and you see the teacher is handing out a test. Oh, no! You forgot to study. It looks like you’re going to have to play it by ear.
To play it by ear means to figure out how to do something while you are doing it. There is no time to plan ahead. The expression comes from when musicians have to play an instrument, but they do not have music to read. They have to use their ears to play correctly. There are many situations in daily life when we have to play it by ear, like going to job interviews or meeting new people.