The Marines refer to any sort of practice as "snapping in." Being a division of the Navy, much of their terminology comes from the parts of a ship. After a day or two, much of this language became clear to me. Here are a few examples:
A "deck" is the floor, so "Clear the deck" made sense. "Overhead" was the ceiling. A "ladder" is the stairs. "Topside" is upstairs.
A "bulkhead" is the wall (At times recruits are expected to put their heels against the wall as a spatial measurement. We were made to do this at the swimming pool).
A "hatch" is a door. Everywhere we went on the bus, a chosen one of us were made to stand by the bus "hatch" and count every person who came aboard the bus. A "porthole" is a window (and also describes a pair of glasses). The DI would criticize the person standing out of formation by calling him a "porthole." I am not sure what that meant except to say that he was "getting in the way."
A "head" is the toilet and a "head call" is using the toilet. We were made to "hydrate" ourselves constantly by sipping water. I was surprised at how many opportunities the DI gave us to use the restroom at least once every half hour it seemed. I was also surprised at how often we needed to use it.
A "squad bay" is a barracks. We were allowed entry into the female barracks where we observed their "racks" (beds) with the neatly made hospital corners.