The ECFMG Certificate is the document ECFMG issues to an IMG who has met all its requirements: an eligible medical school, verified credentials, passing USMLE Step 1 and Step 2 CK, and the clinical and communication skills requirement through a Pathway.
It is required to enter ACGME-accredited residency in the US. It is not a license to practice; state medical boards issue licenses, and Step 3 is usually required for that. If the certificate was earned through a Pathway, it can be subject to expiration unless it qualifies to be made valid indefinitely.
See the ECFMG certification overview for the full list of requirements.