Locked candidates can be hard to spot. In the following image, all of the cells with a candidate 9 have been highlighted in yellow.
The cells marked with a red dot are the only two places where a 9 can go in the bottom left minisquare. One or other of these two cells must therefore be a 9. If one of these is a 9, the cells marked with a green dot cannot contain a 9 without breaking the rules.
We say that the 9 is locked in the left minisquare and can safely eliminate 9 as a candidates from the two cells marked with a green dot.