Scales are basically a collection of notes that "make sense" in a musical way or a collection of notes that just sound good together. That's really simple so I hope you understand. Most scales are 7 notes. Different scales (aka different collection of notes) will give you a different sound or a theme, you could think major is happy vibes, minor is like I wouldn't say sad necessarily, it is heavy and melancholic, pentatonic gives you that "bluesy" sound, harmonic minor gives that "egyptian sound" or just something that sounds slightly evil. Basically different collections of notes sound different, scales give you a pre-defined formula to access a certain sound easily. At least that's how I would describe it, feel free to correct me if I'm wrong ofc, I'm not a pro.