Musicians should learn to identify and play/sing intervals on demand.
The semitone is the smallest interval in Western music. Example: C-C#, F-E, G#-A. On a piano this is achieved by playing the neighbor note, whether white or black. On a guitar it is the next fret up or down that produces a semitone.
A tone is equal to two semitones. Example: C-D, F-Eb, G#-A#. On a piano, just play up or down two semitones (skip the middle one). On a guitar play up or down, skipping one fret.
Here are two videos showcasing the Chromatic and Whole Tone scales.
Chromatic and Whole Tone Scales in C (mid-range voices) Watch on Youtube
Chromatic and Whole Tone Scales in G (low voices) Watch on Youtube