Well, how far back do you want me to go to explain?
A metronome is a device that makes a loud clack or tick or clang with regular timing. You can set it at any speed (within reason). When people who're learning how to play music first try to play along with a metronome, they generally have GREAT difficulty in keeping their notes in time with the regular ticking of the metronome.
So play whatever it is, scales, arpeggios, various patterns, in time with a metronome. If you have no trouble matching the timing of what you're playing wiht the ticks of the metronome, you're doing fine. If you find the metronome appears to be speeding up and slowing down - well, 'tain't the metronome, it's you, you're speeding up and slowing down, and you need to work on your time.
I can always tell the person who never practices with a metronome and sits at home playing - their time is ALL OVER THE PLACE. Most common is the phenomenon where they get to a technically-challenging passage and sllloooooowwwww doooowwwwwn, making sure to play every.single.note.in.the.pattern, but none of them where they should be. This is especially infuriating when the person in question is a guitarist who's supposed to play an introduction that leads all the rest of us into the tune; and when they finish the intro passage no one has a freaking clue what the tempo is supposed to be.
Thus my statement that the acid test of time is playing with the metronome. There are all sorts of variants on this - for example you can set the 'nome on just 2 and 4; you can set it on just 4; you can set it to click on every other 1; the longer the time between metronome clicks, the more likely it is that you will drift away from acccurate time.
Until you can play with accurate time, you can't reliably stretch and rubato things.
Practice with the metronome!