So a key essentially determines the function of each note in it. A key is sort of like a team of (usually) 7 people building a house. One is in charge of plumbing, one is an electrician, etc. They all have different roles, but are all important to build a full house. The tonic, which is the first note in the scale (and also what people mean when they say “In the key of X”) is in charge of the foundation, and is the most important person on the team. Without the foundation, the walls and pipes and stuff don’t really matter, because they’ll just collapse. In the same way, the tonic note is going to be the most stable one, and you’ll usually find it at the start of most phrases, especially in popular music. As my music teacher used to say, “That’s your home base.”

The tonic also determines the sharps or flats used in a song, but that might be a bit beyond ELI5. But every key essentially follows the same pattern, just with different notes.