Limit screws.

The limit screws adjust how far sideways the derailleur can move.

The lowest limit screw sets the limit for the largest cog (lowest gear)

And the highest limit screw sets the limit for the smallest cog (highest gear)

If you turn the limit screws clockwise the derailleur will move inwards and anticlockwise will move the derailleur outwards.

Adjust the limit screws for both the smallest cog and the biggest cog making sure the derailleur doesn't go too far and hit for example the spokes.

One of the best way to do this is to set to the highest or lowest (whichever you are adjusting) gear.

Example below is for adjusting the smallest cog.

Turn the pedals and turn the limit screw so the derailleur is in the gear before the smallest gear

Next turn the pedals and turn the limit screw so the chain just jumps and sits nicely on the smallest cog.

After this is done the limit screws adjustment is finished and the next step is to adjust the barrel adjuster.