1. To attach snugly on(to something), creating an audible snap when doing so. All these pieces just snap on, so you can customize your toy robot to look whatever way you like. I bought a webcam that snaps on my monitor to make video calls a bit more manageable.
2. To cause or force something to attach snugly on(to something else), creating an audible snap when doing so. In this usage, a noun or pronoun can be used between "snap" and "in." I just need to snap this final piece on, and my model will be complete! The soldier snapped on a scope and aimed the long-distance rifle at his target.
3. To turn on some electronic device, especially a television or radio. In this usage, a noun or pronoun can be used between "snap" and "on." Would you mind snapping the TV on? My favorite show is on and I want to catch the latest episode. He shuffled over to his armchair and snapped on the radio.
snap something on
to attach something to something else, causing an audible snap. Dawn took two pills from the bottle and snapped the lid on. she snapped on the lid.