While an LPR (green card holder) is permitted to live and work in the US permanently, the card itself has an expiration date and must be periodically renewed. Green card renewal requires an LPR to:
Submit an application and fee.
Undergo security and background clearances that allow the USCIS to review the LPR's continuing eligibility for the status.
LPRs who are aged 18 and older must carry their green card at all times (INA § 264(e) (8 U.S.C. § 1304(e))). The green card may be used by the bearer as evidence of their LPR status for:
Identity and employment authorization.
Return to the US from travel abroad that has lasted less than one year.