Tension.......
Is the number of stitches and rows measured in cms. The knitted garment is designed by mathematical calculation based on these measurements, so working to a tension that is different from the one statedin your pattern will produce a differently sized garment.
Tension determines the size of a finished garment.

Many knitters don't realise the importance of tension and then are annoyed when their finished garment is too small or the body is too long.
To work a tension square should take no longer than an evening were as knitting a garment can take months. So it is important to work a tension square before you begin so that the finished garment is correct to your desired size.

To work a tension square...
Cast on the number of stitches stated in the Tension part of the instructions, plus 6, keeping the first and last 2 sts knit on every row, this helps the edges to lay flat.
In this instance 28 sts. Work the square in stocking stitch until it measures 14cm e.g.
1st row: knit
2nd row: k2, purl to last 2 sts, k2
Rep last 2 rows until square measures 14cm ending with a 2nd row. Break yarn off and slip it through the stitches and take them off the needle. Don't cast off as this can distort the stitches.

To count the stitches....
A stitch makes a v shape and it's these 'v's that you count as one stitch.
Lay the swatch flat and place a ruler horizontally on the square. Place a pin at the side of one v and another pin 10cm away.
Count the stitches between the two pins, including any half stitches.

To count the rows....
As a stitch makes a and it's these 'v's that you count vertically as rows.
Lay the swatch flat and place a ruler vertically on the square. Place a pin at the bottom of one v and another pin 10cm away.
Count the v shapes between the two pins, including any half stitches.

For our example, if you have 22 stitches and 30 rows between the ruler marks than you have the correct tension and can continue to follow the pattern.

To adjust the tension....
If you have too few stitches, e.g. 21, 20,19 etc, your knitting is too loose and the garment you knit will be larger than stated. Change to smaller needles, e.g 3.75mm.

If you have too many stitches, e.g. 23, 24,25 etc, your knitting is too tight and the garment you knit will be smaller than stated. Change to larger needles, e.g 4.5mm.

If you are finding it difficult to obtain both the correct row and stitch tension it is more important to achieve the correct stitch tension

Examples of garments produced to the wrong tension...
You would like to knit for bust 36" the tension is 22sts but you have a tight tension of 24sts.
Your friend has a loose tension of 20sts and is a size 34"