Right. Or, perhaps in simpler terms, intonation means that when you play fretted notes, they just as much in tune as open string notes. The most common thing that throws off intonation is a poorly adjusted bridge. If the bridge is in the wrong position or the saddle is shaped incorrectly, that changes the position of the string's halfway point relative to the frets, throwing off intonation. Also, when the action is too high (strings too high off the fretboard, caused usually by a saddle or nut that's too high or a bowed neck), the greater distance required to push the string down to the fret puts more tension on the string, making the notes sharper. The easiest way to check your intonation is to compare the pitch of the 12th fret harmonic to the fretted 12th fret note. If the fretted note is sharper or flatter than the harmonic, the intonation is off and needs to be adjusted.
From my experience, there doesn't seem to be a lot of wiggle room for adjustments on ukes. Really you should check intonation when you're buying a uke, and if you already have a uke, it can be adjusted, but really, if you have like, a $50 uke, it probably cant be adjusted to a perfect level if it's already way off, so you may as well just get a more expensive uke, which will probably already have better intonation and action to begin with. (Except Ohana. They're too stupid to check for good action, even on their very nice $300 instruments...)