A scale is a collection of notes which are commonly used together, in order of pitch.
(C major scale ascending)
(A melodic minor descending)
The first example spanned one octave, while the second example continued on, repeating the notes for a second octave.
If you want to compose music, improvise, or write songs, you can choose a key and a corresponding scale, and then use notes from that scale for your tune (melody) and chords (harmony). It’s like your ‘palette’ of notes to choose from.
Major scales and minor scales are the most common types, and there is a major scale for every major key. Minor scales are slightly more complicated (they come in 3 flavours: melodic, harmonic and natural) but they correspond to the minor keys in a similar way.
Modes are a special type of scale where the notes are chosen to suit a key (like with major and minor scales), but then they are put in a different order (by starting the sequence from a different note).
Here are two modes which use the same notes as the C Major example above:
(D Dorian mode)
(E Phrygian mode)
Can you hear the different musical mood they create just by starting on a different note?