This music theory lesson is about tones and semitones.
More resources on this topic can be found at
https://www.musictheoryacademy.com/understanding-music/tones-and-semitones/
In this video, the definition of a semitone is explained with reference to a piano keyboard, showing that a semitone is made up of 2 adjacent notes on a piano.
The lesson also shows how there are 12 semitones in an octave and that these make up the chromatic scale.
Tones are also explained in the lesson as being an interval made up of 2 semitones. The Whole Tone Scale is also introduced and audio examples and sheet music examples given.