Additional filings are required to maintain the registration of a trademark. If you do not make these filings your trademark will be considered abandoned by the US Government and all of your federal registration rights will be lost.
Specifically, there are filings due between the fifth and sixth year of ownership, at the tenth year of ownership and every ten years thereafter. The US Government will not notify you of these requirements. It is completely your obligation to make these filings.
We can also file a §15 affidavit to have your trademark declared “Incontestable”, which makes it much harder for opponents to attempt to challenge your registration.