. . 4 | . . . | . . .

. . . | . . . | . . .

. . . | . . . | . . .

------+-------+------

. . . | . . . | . . .

. 4 . | . . . | . . .

. . . | . . . | . . .

------+-------+------

5 . . | . . . | . . .

x . . | . . . | . . .

. . . | . . 4 | . . .

At the position x according to a technique the only solution is 4.

Unique Candidate

You know that each block, row and column on a Sudoku board must contain every number between 1 and 9. Therefore, if a number, say 4, can only be put in a single cell within a block/column/row, then that number is guaranteed to fit there. This example illustrates the number 4 as the unique candidate for the cell marked.

I don't understand why that is the case.