Full employment refers to the period in which there are plenty of jobs in the market and thy are filled with people who want them, hence abundance of job opportunities are there in the market.
In simple term full employment means jobs for all that want them. This does not mean zero [unemployment] because at any point in time some people do not want to work. Also, because some people are always between jobs, there will usually be some [frictional unemployment]
Full employment means that everyone who wants work and is willing to work at the market wage is in work.