Job supply is, broadly speaking, the number of jobs available. It is what is normally intended when people talk about jobs being created, cut, destroyed, and so forth. It is effectively the opposite of job demand.
Depending on the nature of the work, this might be expressed more precisely as either (a) the number of additional work-hours an employer needs per week or (b) the employer's unused human resources budget.
When there are more people looking for work than there are job openings, as is almost always the case in a capitalist economy, some people will be unable to find work.