Hello,
If you are talking about the root key of a sample, this
page will explain how to configure it. At the sample level, the goal is to determine the exact pitch of a sample.
Once the sample is configured, you can then add it in an instrument and define a range, which is usually around the rootkey of the sample. With experience, I prefer using a keyrange 28-40 for a sample having a rootkey of 40 rather than using a keyrange 40-52. The reason is that when the sample is resampled to a higher pitch, high harmonics may be amplified and be annoying.
Use the parameter "override rootkey" at the instrument level only if you want to transpose the sample in the instrument. For example, if you specify that the override rootkey is 52, the corresponding sample will sound with no resampling with the key 52. If the original rootkey was 40, you just transposed it an octave below. For a simple instrument you shouldn't have to use it.
