If two or more things interlock, they fit into each other so firmly that they hold. Think of puzzle pieces, of fingers pushed together, or of bricks laid so each one sits in the next. Machines interlock when one part will not move unless another part is in the right place — a door that will not open until a switch is on, for example. People stretch it a little for plans or ideas that fit tightly, but the home meaning is physical: they lock into one another, not just sit side by side. It is more precise than “fit” or “join,” and a bit more formal. Though actually, you will also hear interlocking as a describing word (interlocking bricks, interlocking fingers); the meaning here is the verb: they fit firmly together.