Unemployment refers to not having a job. The more people have work, the less unemployment there is.
When you're employed by someone, you work for them: employment is a job. Therefore, unemployment is the lack of a job. Economists often talk about how much unemployment there is and whether the unemployment rate is increasing or decreasing. If it's up, that's bad for the economy, because more people are out of work. Sometimes, the government provides unemployment benefits for people who have recently lost their jobs.