The two basic elements of music are pitch and rhythm. "Pitch" refers to the perceived highness or lowness of a note. "Rhythm" refers to the pattern in time made by a series of notes. Standard music notation provides a simple way to represent both pitch and rhythm.
To understand pitch notation it will help to understand the concept of the scale, a set of pitches arranged in a pattern of small and large musical steps. There are various types of scales, each of which follows its own pattern. Standard pitch notation and the white keys of a piano keyboard both evolved to represent the pattern of large and small steps contained in the notes A,B,C,D,E,F,G. When those notes are played starting with C, the pattern is called the major scale, the scale of much familiar music.
The small and large steps that form the major scale pattern are called the half step and the whole step. A half step is the smallest distance between two keys on the piano; a whole step is equal to two half steps. A white key to an adjacent black key is always a half step, but not all white keys are a whole step apart. The piano's white keys contain two half steps: E-F and B-C, highlighted on the keyboard below. As you have probably noticed, there is no black key between white keys a half step apart, since a half step is the smallest step possible on a piano. The best way to explain these is to let you hear them.