There is a naked single when an empty cell can only hold one number: the other eight are already in its row, its column or its box. That is the number missing from the cell.
Unlike the hidden single, with the naked single you look for the number missing from a cell, instead of looking for the cell where a number goes. It is called naked because, with the notes written, the candidate is in plain view: it is the only number noted in the cell.
An example
There is no hidden single in this grid: scanning number by number, box by box, finds nothing. So let's look at the cell in row 9, column 8, with the three units it belongs to.
Row 9 already has the 3, 4, 6 and 9. None of them can go in this cell.
Column 8 adds the 2, 5 and 8. It also has the 4, 6 and 9, but those were already counted.
Box 9 adds the 7. Eight numbers are ruled out. The cell's only candidate is the 1.
The 1 is this cell's number. Notice that, in this grid, the 1 appeared only once: anyone scanning for it would always find more than one possible place in each box.
That is why the naked single turns up when scanning runs dry. Scanning looks at one number at a time; the naked single brings together in one cell the information from several different numbers, coming from three units.
The last number
The easiest case of all is a unit with a single empty cell. It is missing one number and there is only one place for it. It is called the last number, also known as a Full House, and it is the first thing the game's hints look for.
Row 2 has eight numbers and one empty cell, in column 4. From 1 to 9, the one missing is the 1.
The 1 goes in the only free cell of the row. There is no need to look at anything else.
The last number is both a naked single (the cell has only one candidate) and a hidden single (the number has only one cell). You can see it without counting: just notice a row, column or box that is almost full.
How to search
With notes, go through the grid looking for cells with a single number written in them. With the notes up to date, there is nothing else to do.
Without notes, start with the cells that have the most numbers filled in across their row, column and box: they are the ones with the fewest possible candidates.
After placing a number, look around. The new number is removed from the notes of the 20 cells it sees. One of them may in turn be left with a single candidate. Naked singles often come in chains.
A cell with no candidates is a warning. It means that somewhere there is a wrong number or a note removed by mistake. It is better to go back than to carry on.
Hidden or naked?
With these two searches (the cell where a number goes, and the number missing from a cell) you can solve any Easy puzzle. The order in which to do them, and what to do when neither gives an answer, are in where to start.
Your turn
First with the notes in view: find the cell with a single candidate and place the number.
Now without notes. There are no hidden singles in these positions: you have to choose a cell and count the numbers it sees.