This strategy is based on correctly placed notes. Sometimes, it is also called "Last Digit." The key is that within a specific cell, there is only one possible number left (from the notes).
Let's look at an example of this situation.
Look at the highlighted cell. We can see it has only one note written in it: the number 2. This means there is only one possible solution for this cell. Since this is the only possible option, the value for this cell will be 2. Therefore, we remove the note from this cell and fill in the number 2.
This is the principle of the "Obvious singles" solving method. As you can see, it's not as difficult as it might seem. If you put the "Obvious singles" method into practice, the process of solving Sudoku will become much easier and faster!