Colloquial language means informal spoken language, i.e. language as it is used in everyday life and in informal situations.
Colloquial language differs in particular from written language in that it is has a far less formal register. It is also less formal than some other forms of spoken language. This means among other things that it has a specific lexicon.
Because colloquial and written language are in general considered variants of the same language, the distance between them is usually not very great, except in situations where diglossia exists, as for example in the Arabic language.