In the set up menu activate COURSE DEVIATION INDICATOR (CDI). You can set the amount of drift, when activated the CDI will have a deviation measurement bar horizontally across the compass page. The CDI indicator is a vertical line which moves to depict course deviation and where the intended track lies relatively to your current position. If for example you drifted left the CDI line will show the bottom of the line swing out to the right depicting you are now left of track. As you apply a right turn correction and begin to intercept the correct ground track again the bottom of the line will swing back towards the centre.
This is a very useful tool, if you had planned a course from one way point to another which was to say take you through a gap between two high features with a distance of 400m between them. If you have drifted off course the GPS will continually update the course to take to get to the waypoint, but this new heading may now become controlled flight into terrain in bad weather. By using the CDI you can course correct as soon as possible and know that it will take you safely between the intended path between the features. Although no one advocates flying in cloud or poor visibility, the CDI will at least follow the intended ground track until you get out of it. For those PPL guys this is the same system as flying radial on the VOR system.