Nesting is the concept of having a thing under another thing. 1+2 will be referred to as a mathematical equation and 1+((((a-b)/3+2/87)-6)23) will be referred to as a nested equation. Of cause this is rather lose semantics since both are expressions or statements and not equations, but that’s how the language is typically used.
This is what that looks like in wolfram alpha: https://www.wolframalpha.com/input?i=1%2B%28%28%28%28a-b%29%...
It looks the same in Mathematica while your typing it in, which makes I extremely easy to work with, because even very complex equations look exactly like usual in Mathematica instead of ending up looking like lisp implementation of the Fibonacci sequence.