`With` substitutes constant values into the body expression.
With[{x = x0, y = y0, …}, expr]
With[{a1 = …, a2 = …, …}, {b1 = …}, …, expr]
With[{x = 5}, x + 1]
→ 6With[{x = 2, y = 3}, x + y]
→ 5With[{l = Length[{1,2,3}]}, l + 1]
→ 4With[{x = 5}, {y = x + 1}, y^2]
→ 36With[{x = 5}, {x = x + 1}, x^2]
→ 36