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 1.Nxf6+. Black has to recapture with 1...Qxf6 (1...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 1...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 knight with 1...Nh3+, clearing the way for the queen to g5. If White captures the knight with 2.Qxh3, the queen is deflected from protecting the rook and Black can win with 2...Qxd1+ 3.Re1 Qxe1#. If White plays 2.Kh1, Black wins with the fork 2...Nxf2+ 3.Kg1 Nxd1. And if White recaptures with 2.gxh3, the g-file is opened and 2...Qg5+ comes with check and picks up the unprotected rook, for example 3.Kf1 Qxe7. 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.