Half steps are basically what they sound like. Half a step.
One half step is always the closest available note. The
simplest way to to illustrate this is with a keyboard. If we
start on middle C, and move up to the closest note (Db, in
this case a black key) we have a half step.
Intervals are built on half steps from 1(unison)-8(octave)
and in some more advanced cases keep going.
C - Db