Welcome to the forum!
It’s explained on page 31 of the rule book. If you don’t have the rule book, check this thread for a download: http://www.axisandallies.org/forums/index.php?topic=16910
The rules say: “Loading onto and/or offloading from a transport counts as a land unit’s entire move [….]”
Not the “and/or”: the land unit may load and offload in one move, but it may also choose to load, and offload at a later time.
Also: “A transport can load cargo in friendly sea zones before, during, and after it moves. A transport can pick up cargo, move one sea zone, pick up more cargo, move one more sea zone, and offload the cargo at the end of its movement […]”