Dashboard Brake Light Comes On. Why?

Lost my manual decades ago, so can't just look it up.....but does anybody know what the round red brake light on your dash means? I am guessing engine vacuum, as it only seems to come on when you are about to stall or stalled. It came on yesterday after driving for first time this year, and the brake pressure felt soft. It went off again and home I went, but I have no idea how to diagnose.

Also, while I am at it, I have been wanting to purchase a devise to check my vacuum. Any suggestions?
Thanks

You said the brake pressure felt soft. If it’s the brake light then you have an issue in your brake system