Could be an air lock, could be impending brake servo failure. Or any of a number of potential other problems.

I would not use it on the highway until rectified. You have a life-threatening defect on the car that you now know about. Using it in such a state would invalidate your insurance and leave you open to criminal charges.

When was the brake fluid last changed?

If the fluid is old, this could be boiling brake fluid, but that's only likely if there's also something causing brake heating, such as the (learner?) driver "riding" the brakes, binding brakes, stuck handbrake cable etc.

If this is the case, changing your brake fluid will stop it happening, but you may still have other brake problems, perhaps due to corrosion caused by the old fluid.

If you're told its something else (faulty master cylinder seems to be popular) it may be, but it'll be hard to tell since they'll also change the brake fluid when they replace it.

I had a foot-to-the-floor brake failure a few years ago which I was very confidantly told was a faulty master cylinder, and it may have been, but replacing the brake fluid fixed it and the master cylinder is still there.