FL Studio's default is C5.....so every note you play....is relative to, or in tune of the key of C.
Some samples are TUNED differently..
For instance, a kick drum.....some samples, for example.....if you play them on the note of C....the sample itself is playing, lets say F.
Its out of tune.
So in that case, you would right click the F note...... So when you finally do play the sample on the note of C...its actually playing a C note...meaning it is now in tune.
There are articles, tutorials, and videos all over the place about tuning drums and instruments in FL Studio.....most of them will answer your questions relating to this thread...and probably explain it better than me lol