@kitten früh means early so Frühling is the early season of the year. Früh is also sometimes used for morning like "in der Frühe" or "frühs". Stück means piece so Frühstück is an early piece (of food) -> breakfast
@kitten "Früh-" meaning "early " "Stück" means "piece" but here probably short for "Süßesstück" which is a synonym for "Teilchen" which translates to "bun". "Frühling" cause it's the season that is early in the year. "Frühstück" means the piece/bun you eat early in the morning.
@kitten Stück is a piece. Früh means early. Centuries ago, Frühstück used to be the piece of bread that you eat in the early morning. Frühling is the early part of the year.