"Forced Digit" Sudoku solving technique explained:

sudoku demonstrating the forced digit technique

A "Forced Digit" means that in a specific cell only one number remains possible.

This applies after eliminating all the other candidates by using different solving techniques. In other words, a forced digit is the last remaining option and this is why we are forced to put it there. Thus it's called a "Forced Digit"

Take a look at the example:

look at the block marked in pink, the cell marked with a blue square has only option - 6

sudoku demonstrating forced digit solution

It’s also known to some by the names "Sole Candidate" or Naked Single"