"Internal time" is the sense of the beat that you have when there isn't something providing a reference, like a metronome, drummer or other accompaniment.  If you have to tap your foot when you're playing alone, to stay on the beat, some might call that internal time, but a lot of teachers and musicians won't. 

Like your intonation or anything else, it can get better with practice.  I ran across some fun(?) drum tracks for practising it.  This one is a "3+1", meaning that after it gets going, the drums will play for 3 bars and then be silent for a bar.  If you are right there on the beat when the drums come back at the start of the next measure, then your internal time is doing good. 

These are about 10 min or so long, which is enough for a nice workout/exercise to work on your internal time.  They're available in different tempos, if you want to maybe start a "program" with working at a different tempo every day or week or whatever.  Or if there's some piece you're working on polishing, doing your scale or arpeggio practice at the tempo (and key) of the particular piece can make the rest of practice go a whole lot easier since you'll be "warming up" in it before working on the piece.

This sort of exercise maybe isn't everyone's cup of tea, but I figure some of us here work on such things, so I figured I'd post it.

Bon apetit!