The step up on the word "birth" in "Happy Birthday" is a major second. The first two notes of the Jaws theme are a minor second apart. Same idea, a different distance, and a very different feeling. That distance between two notes is called an interval, and a lot of music theory is built from it.
What an interval is
An interval is the distance between two notes. C up to E is a major third, and G up to B is also a major third. Same distance, different starting note. You measure it by counting half steps (semitones), which on a piano are the distance between neighbouring keys, black and white included.
The common intervals
Minor second: 1 half step. Major second: 2. Minor third: 3. Major third: 4. Perfect fourth: 5. Tritone: 6. Perfect fifth: 7. Minor sixth: 8. Major sixth: 9. Minor seventh: 10. Major seventh: 11. Octave: 12.
Why they matter
Scales are fixed patterns of intervals. The major scale goes whole, whole, half, whole, whole, whole, half. Chords are stacks of intervals: a major triad is a major third with a minor third on top, and a minor triad flips them. Melodies are strings of intervals, and recognising them by ear is a big part of ear training.
Ways to remember them
Link each interval to a song you know. A perfect fifth is the first two notes of "Twinkle, Twinkle, Little Star". A perfect fourth is the start of "Here Comes the Bride". An octave is the first jump in "Somewhere Over the Rainbow". The reference songs do the work for you.
Consonant and dissonant
Perfect fourths, fifths, thirds, sixths and the octave sound stable. Seconds, sevenths and the tritone sound tense and want to resolve. That tension is a tool: it is what makes a dominant seventh chord pull back to the home chord.
My full guide has exercises and a longer list of reference songs: Intervals Explained: The Building Blocks of Melody and Harmony.
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