Explanation

Clearance is similar to a discovered attack or discovered check, in that you move one piece to reveal a threat of another piece. For a discovered attack or check that threat is a direct attack or check. For clearance, that threat can be checkmate, or some other tactics.

A clearing move can be especially strong if it is a forcing move, for example a capture, a check, or an attack. In that case, the opponent often cannot handle that forcing move, and the revealed threat at the same time.

In this level the revealed threat is a fork. You move one piece out of the way for another piece to give a fork on a subsequent move.

Examples

White can exchange knights with Nxf6+.
Black has to recapture with Qxf6 (gxf6 doesn't help and is even worse).
The knight has cleared the way for the d-pawn to d5, which forks Black's knight and bishop.

Sometimes the clearing move can be a sacrifice:

Black can sacrifice the exchange with 1...Rxd4.
The rook has cleared the way for Black's knight to f4, so if White recaptures with 2.cxd4, Black wins a rook back with the knight fork 2...Nf4+.
The sacrifice was a good investment, in the end Black is a knight up.

Note that after Rxd4 the counter attack 2.Rxg5 does not work, for example 2...Nf4+ 3.Kh2 Rc4 4.b3 Ne2, a nice series of counter attacks.

Sometimes clearance can involve other motifs, for example opening a line, deflection, and decoy.

Black can sacrifice the exchange with 1...Rxe3.
The rook clears the way for the queen to d1.
White has to recapture with 2.Qxe3, which deflects the queen from protecting d1, and from protecting the rook.
Black can now win the rook back with the queen fork Qd1+.
If you can visualize such combinations involving multiple motifs correctly, you are on a good way to master tactics.

Related motifs

With clearance, you move one piece to reveal a threat of another piece.
If that threat is a direct attack, it is called discovered attack, or discovered check.
If you force an opponent's piece to move out of the way of your piece, it is called opening a line, or using the pin in case of a direct capture.

You can also combine clearance with other motifs: