A Naked Single is the first technique every Sudoku player learns. Pick an empty square and look at its row, its column and its 3×3 box. Every number already written in any of those three places is ruled out for this square. If eight of the nine numbers are ruled out, the one left over is the answer.
Beginners often find naked singles by scanning squares that sit in a crowded row and a crowded column at the same time. The more numbers around a square, the more likely it has only one option left. Pencil marks make naked singles easy to spot, because the square will show just one small number.