A module is an individual part that combines with other components to make different things. You might have one module that's a cube, and another that's a slab of wood. Two cubes and a slab could make an end table.
In space exploration, module can refer specifically to one of the lunar modules used for landing on the moon during the Apollo space missions. Two astronauts used a module to descend to the moon from the orbiting Apollo spacecraft and to return when the moon mission was complete. More recently, the Space Shuttle delivered several modules to the International Space Station, which is built on a modular design.