Maven lets you specify either goals or lifecycle phases on the command line (or both).
clean and install are two different phases of two different lifecycles, to which different plugin goals are bound (either per default or explicitly in your pom.xml)
The clean phase, per convention, is meant to make a build reproducible, i.e. it cleans up anything that was created by previous builds. In most cases it does that by calling clean:clean, which deletes the directory bound to ${project.build.directory} (usually called "target")