It effectively excludes the entire C: drive from being scanned for malware. On my laptop, I see that some McAfee antivirus folders are excluded. Likely because those folders might contain virus signatures that McAfee uses to detect malware. That exclusion prevents Defender from falsely detecting the McAfee files as viruses.
PS C:\> (Get-MpPreference).ExclusionPath
c:\program files\mcafee'c:\program files\common files\mcafee'c:\program files (x86)\mcafee'c:\program files (x86)\common files\mcafee'C:\DELL\FD09N'c:\programdata\mcafee
PS C:\>