A hidden single is easy to walk straight past. The cell may have three or four candidates written in it, so nothing about it looks decided. But if you scan the unit for one particular digit and find it written in only that cell, the digit has nowhere else to go.
The other candidates in that cell are irrelevant — they were possibilities for the cell, and this is a deduction about the unit. Write the digit and erase the rest.
This is the first technique where notes genuinely earn their keep. Without candidates written down, a hidden single is invisible; with them, it is a scan of nine cells for one symbol.