In English we add 'er' at the end of the word to form a comparative (i.e. pretty -> prettier, shallow -> shallower).

In Latin you add 'ior' at the end for the masculine and feminine forms, or 'ius' for the neuter (i.e. longus, longa, longum -> longior, longior, longius meaning long -> longer).

The context for a comparative would be exactly the same as in English, for example 'this boy is taller than that boy'.