For a list of what each Windows Server Functional Level provides or changes see: https://learn.microsoft.com/en-us/windows-server/identity/ad-ds/active-directory-functional-levels

As your bring a new Windows Server into your domain and when you make it a Domain Controller Role handler you may need to raise your Domain or Forest Functional Level.

• First you should review the effects of raising the functional level.
• Review if you have any older servers that are Domain Controllers or hold that role, that raising the Functional Level would cause issues with.
• Review if older servers in the Domain or Forest could be unable to participate in the domain.
• If you have servers older than Server 2008, convert from FRS (File Replication Service) to DFS (Distributed File Service) – https://techcommunity.microsoft.com/t5/storage-at-microsoft/streamlined-migration-of-frs-to-dfsr-sysvol/ba-p/425405 Windows Server 2016 is the last version to include FRS.
• To Check your Forest and Domain Functional Level – open Active Directory Domains and Trusts, right click on the Domain or Forest name below Active Directory Domains and Trusts and choose Properties.
• To Raise the Forest Functional Level – open Active Directory Domains and Trusts right click the Domain or Forest name you want to raise and choose Raise Domain Functional Level or Raise Forest Functional Level.
  • To Raise Forest Functional Level – right click on Active Directory Domains and Trusts and choose Raise Forest Functional Level
  • To Raise Domain Functional Level – right click on the Domain name right below Active Directory Domains and Trusts and choose Raise Domain Functional Level.
• This will display the Domain or the Forest Name and it’s current domain functional level, and a drop down to choose the new functional level you want to use. As this dialog displays be aware that “After you raise the domain functional level, it is possible that you may not be able to reverse it. For more information on domain functional levels, click Help.” https://learn.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2008-R2-and-2008/cc754217(v=ws.11)?redirectedfrom=MSDN
• Lastly do not forget about other integrations with your Active Directory Domain/Forest and how they may be affected, think of SAMBA on Linux and other Network Storage devices, or cloud services.