A "buckle" is the part of a belt or strap where you attach the two ends to each other. The belts people wear on their pants have buckles. So do some shoes, straps for handbags and luggage, and seatbelts.

When you attach something with a buckle, you're "buckling" it. You "buckle" the strap or belt itself:

Can you buckle that strap?

And you "buckle up" or "buckle in" the thing that's contained by the belt or strap:

There you go; you're all buckled in!