A proof of service (or certificate of service) documents that a filing was properly delivered to the other parties in a case. It's a short document, but courts treat it as essential — without it, the underlying filing may not be considered properly before the court at all.
What it needs to state
Who served the document, what was served, how it was served (mail, personal delivery, electronic filing), when, and to whom — each element needs to be accurate and specific, not just a general assertion that service occurred.
Match the method to what's actually allowed
Different documents and different case types allow different service methods — some require personal service, others allow mail or electronic service. Using a method the rules don't permit for that document can invalidate the service entirely.
Sign it under penalty of perjury
Most jurisdictions require the proof of service to be signed under penalty of perjury by the person who actually performed the service — not just filed as a routine attachment.
File it with, not after, the underlying document
Proof of service should generally be filed alongside or promptly after the document it certifies — a long gap, or a missing proof entirely, can create real doubt about whether proper notice was given.