Usually if that happens it means one of a few things.

If it is intermittent and not accompanied by other symptoms, it may be because of a high protein content in the urine. This is pretty common, and can happen for a few reasons, but sometimes it just happens. If it happens super frequently (several times a week, for example), then get it looked at because it can be an indicator of bad kidney function.

If the froth is constant for a long period of time, it could indicate a UTI, even without other symptoms. If you have no other issues, you could try taking cranberry pills and see if it clears up. Again, if it keeps happening repeatedly, see a doctor just to be safe.

Last option is you could have an STI, but if you have no other symptoms and no reason to suspect exposure, then you can safely rule that out.

Source: I get this occasionally too and have nurses comment on it whenever I need to give a urine sample for various tests.