These terms origin, belly, and insertion have to do with both the anatomy of a given muscle and with movement of that same muscle. I like to think of the brachialis muscle during an arm curl as a good example for these terms.
Origin: This refers to one attachment point of the muscle (usually to bone). You can think of the origin of the muscle as its anchor point, when the muscle pulls we think of this end as the end that does not move during the given movement… During an arm curl the origin of the brachialis muscle is the humerus.
Belly: This is the thickets portion of the muscle along its length (this term just has to do with anatomy).
Insertion: This refers to another attachment point of the muscle (again, usually to bone), in this case the muscle is attached to the part of the body that the muscle is moving. In the brachialis and arm curl example the insertion is on the ulna (specifically the coronoid process and ulnar tuborisity). As we perform an arm curl it is the unla (and the rest of the lower arm) that is being moved as the brachialis contracts.
I’ll be coming back here to add some images that will helpfully help clarify what these terms mean 🙂