A common method of increasing stitches in knitting is known as make one, abbreviated as M1 or M1L, for make one left.
The make one is performed in between two stitches, with the bar between the stitches.
To make a M1L or make one left, take the left-hand needle and pick up the bar between the stitches from front to back, as shown in the picture. Use the right needle to knit this bar through the back loop.
There's also a stitch known as the M1R or make one right, which involves lifting the bar from the back and knitting it through the front loop. This one is a little trickier, and patterns don't always specify which one of these stitches they want you to perform (the pattern will just say M1), which means you can use either, and M1L is much easier once you've tried it a couple of times.
The same stitches can also be worked on the purl side, just purl though the back loop for a make one left and through the front loop for a make one right.