Tonic function

In harmony, a chord is said to have a tonic function if it feels "stable" and "at rest", or that it doesn't "feel the need to move to another chord".¹

In diatonic music the chords that have a tonic function are $I$ and $VIm$.¹²

It's worth noting that $I^6$ is not nearly as stable as $I$ and that $I^6_4$ is often actually of dominant function² (also see: cadential 6-4).