Can I set a radius for geolocation?

Can I set a radius on the longitude and latitude values so if you come within 100 meters of the location it validates and unlocks part of the game? Or does anyone have any suggestions around this. The idea is that you can unlock new parts of game if you are in a specified location.

If you know your exact location coordinates, you can then check if you are at a

(PseudoCode)
Longitude > Target.Longitude - Range AND 
Longitude < Target.Longitude + Range AND 
Latitude > Target.Latitude - Range AND
Latitude < Target.Latitude + Range[/code:2uliiual]

If all 4 previous conditions are true, then you are in range of the target coordinates.

I couldnt understand what is this code doing. Can you please elaborate it a bit?