its one of those 'multipurpose' api queries, where the parameters dictate what information you want (for example, thread exit code, thread entrypoint, if a thread is debuggable and so on)..
the threadinfoclass is the 'key' parameter, which isnt documented too well, so like all good reversers, we hope that reactos has a reference.. and.. it does
so all you then need to figure out is the size of ThreadInformationLength required for the threadinfoclass you want is, so you just do it in the usual fashion of calling it with a small buffer, then checking the return value from the call (if it worked, its NT_SUCCESS which is zero), otherwise it'll be another value, typically one indicating the buffer was too small (in which case allocate a buffer of size of the (ReturnLength) parameter you passed and call again...
hope that helps
edit : just remembered this...
ThreadHideFromDebugger is a commonly used anti debug, it has one caveat however, (parameter passed is a pointer to a TRUE (dword)...), once this is done, passing false etc.. will NOT work.. ie: once the call is successful for that thread, the hide from debugger can not be undone)