Passthrough is the ability to move from the RVs living area to the driving area (cockpit). It can also refer to the actual area one passes through, as in a bulkhead.
Passthrough is easiest on motorhomes, step vans, and high roof vans where one can walk from one to the other. It is impossible on trailers where the tow vehicle (TV) is separate from the travel trailer (TT).
On some campers the lack of formal passthrough can be worked around. For example, in a box truck the rear cab wall can often have a portal cut in it. In cars and SUVs one might be able to wiggle to the front (especially if the passenger seat is removed). In low roof vans one can usually stoop to get into the cockpit.
Passthrough can be a factor in both convenience and safety.