A green card is proof that its holder has been granted permanent residence status and has been officially granted immigration benefits. The benefits give the person the right to reside and work in the United States as long as they do not abandon their residence by being outside of the country for too long. If certain conditions are not met their permanent residence status (green card) can be revoked and the party would be subject to deportation.
If you are residing in the US already you may be able to adjust your status to a permanent resident without leaving the country. If you have violated your immigration status by violating a US law or committing a crime other legal steps can be taken to have a conviction vacated and make you eligible to reclaim your green card or lawful permanent residence status.