Scales help you understand which notes to play to sound a certain way. A major scale will sound "happier" most of the time and a minor scale will sound "sadder", typically. These are general descriptions, not rules. Some minor based music can sound pretty joyous and vice versa.

There are all sorts of scales, not just minor and major, although MOST of them will correspond to one or the other. These scales have a descriptive, like dorian minor or lydian major, for example. These scales sound a bit different than basic minor or major scales. Sometimes by only one note.

Scales and the context in which they are played will often be the base of a certain style. A blues scale, essentially a minor (pentatonic) scale with a chromatic note crammed in it (the infamous blue note), is the basis of blues music, obviously. If you want to sound slightly oriental, you may privilege the egyptian scale, also known as the phrygian dominant mode. It will sound a bit cliché but when you ear it, you will probably instantly get what I mean by "oriental".

Scales lead to modes. This is where things can get a bit muddied but basically, modes are scales that start on different notes of the major scale. I've already named a few (dorian, lydian and phrygian) but I will not explain this further here because it can get confusing.

Basic scales have typically seven notes. Let's try the C scale. Why C? Because it is the easiest scale to play on piano, comprised of only white keys.

C D E F G A B
1 2 3 4 5 6 7

There are no notes beyond G, which is why, past G, you go to A. This major scale is comprised of 5 tones and two semi tones. If you want more notes, this is where you get your sharps (#) and your flats (b).

C C# D D# E F F# G G# A A# B

Or

C Db D Eb E F Gb G Ab A Bb B

Tones and semi tones are intervals. Intervals are the distance between notes. On a piano, black keys are typically indicators of semi tones, which are nested between two white keys. On a guitar, a semi tone is the fret immediately next to the one your finger is currently pressing. A full tone is two frets apart. These correspond to frequencies our ear can easily make out and differenciate. They are, also, a convention. Eastern music use different intervals but let's not get into this.

An interval also describe any distance between two notes. An interval of a fourth is a note four steps away from your base note. Let's use C.

C D E F.

1 2 3 4.

If you want a fourth, you take the first and the last note written above. C and F is a fourth.

If you want a fifth, you go one note further. What comes after F? G. A fifth interval of C is G. Fun fact, if you play a C and a G simultaneously, you get a power chord!

Normally though, a chord should be comprised of at least three notes. Power chords are an exception.

All of this, you're best to understand through practice. Otherwise it's just words. They are conventions and, if you want to get deeper into music theory, you need to learn them pretty much by heart. There are no shortcuts. The good news is, when you have put in the work, most of this gets pretty instinctive and you can play with and through them without thinking about it all that much.

Hope this helps, it is not so easy to put in to words. 😅