The block expression is sort of an "invisible" expression, meaning you don't explicitly create them: They are implicitly created whenever you have an indented block. Blocks are essentially a list of expressions that get evaluated one after another, and the last expression of the block is the return value of the block expression.
let x = let y = 123 y echo (x)
Here we are creating an indented block, assigning
y, then evaluating (ie, returning)
from that block, which finally gets assigned to