From UBC Wiki

what are necessary conditions? a condition A is said to be necessary for a condition B if and only if the falsity/non-occurrence of A guarantees or brings about the falsity/non-occurrence of B

Examples of necessary conditions:

To be a bachelor, it is necessary that one be a male.

It is necessary for humans to have oxygen (condition A) in order to live (condition B). Therefore if a human does not have oxygen, they are not able to live.