Colloquial means informal.
Colloquial is a adjective.
Synonyms: casual, familiar, chatty, common, conversational, folksy, homespun, unsophisticated, ordinary, plain.
Antonyms: cultured, formal, sophisticated, refined.
The company suggested a colloquial writing style for the advertisement purpose; the informal language should appeal to the customer.
I had a difficult time understanding the colloquial expressions or clients from German language because I had only studied formal German language.
In ISC many forums posted by new members are written in colloquial style; however such posts are not accepted.