Lambda calculus is the foundation of functional programming, which is why functional programming languages love to include a λ in their logos.
But what is lambda calculus, exactly? You could read the Wikipedia article on lambda calculus, but it’s pretty rugged and arcane — and I’ve linked to some pretty rugged and arcane Wikipedia articles on this blog.
Instead, I’m going to bounce you to the Palmström blog, which posted back in 2012 a great introduction to lambda calculus for non-math people.
This article was the one that made lambda calculus click for me, and helped me get a grip on why stuff like first-class functions and referential transparency are so important in functional programming from a math perspective.