Code Inspection: Type or member is never used (non-private accessibility)
Unused types or members might indicate dead code, which can occur, for example, after changing your codebase. ReSharper detects methods that are not called and types that are not used.
To reduce code size and simplify its maintenance, ReSharper suggests you to either remove or comment out the unused type or member.
Of course, there are a lot of cases where symbols can be used in less traditional ways — via reflection, different frameworks — or they just could be designed as a public API.
You can always suppress usage-checking inspections locally with a comment, but the recommended way is to decorate unsused symbols with code annotation attributes. There are two attributes for this purpose: [UsedImplicitly] and [PublicAPI], which are functionally similar, but let you and your teammates understand how the symbol is actually used.