Chromatic scale is a particular scale where , all the notes are always in semitone.
What is semitone?
If there is no note between two notes it will be a semitone and if there is one not between two notes it will be a tone. You can say, it is a relationship between two notes.
C and D is a tone because in between that, there is a note that is C Sharp or D flat.
E and F or B and C is a semitone because there is no note between them.
F sharp and G is a semitone. F sharp and G sharp is a tone.
Example of a chromatic scale:
C C# D D# E F F# G G# A A# B C
There is lots of ways to write down a chromatic scale in stave notation
we are not discussing that thing in here.