An interval is the amount of space between two notes. A half tone is the smallest space between two notes. C and C# for example is a half tone/half step. Also sometimes called a semitone (mainly in Europe).
Here is the same idea, but written in Treble and Bass Clef.
How two notes interact together is known as harmony. If they sound pleasant to our ears, they are known as harmonius. If they are tense, or do not sound pleasant, it is known as dissonant.
Many composers use dissonant intervals to set up tension before resolving to a harmonius interval to release that tension. This adds interest to music.