the adjective form of 'certain' is used to describe something of which one is certain. In this context, it essentially carries the same definition as the noun form – something definite and with no doubt.
when used as an adjective, 'certain' usually precedes a noun (the 'what') that names what one is certain of. It is typically used to describe how sure of something one is, or how definite the information at hand may be. For example
1. I am certain of the date and time of our meeting tomorrow.
2. We have a certain level of confidence that the project will be successful.
3. I am certain that his story wasn't true.
as with other uses of 'certain,' be aware of context and tense. When referring to a single item, other adjectives such as 'sure' or 'definite' may be more appropriate.