What does “time served” mean?

Chelsea has applied for clemency and is requesting that her sentence be “commuted” to “time served”.

This means that her punishment would be deemed to have been completed by the already 6 1/2 years that she has served.

The time that Chelsea has already served in prison is longer than any individual in United States history charged with turning over documents to the press to benefit the public interest.

What does “time served” mean?