A common analogy for how "blocks" work in a workflow is the way that brackets (like this!) work in a sentence. Brackets have an explicit start point '(', which is always matched by a specific end-point ')'. They demarcate an area of the sentence that has a specific meaning.
The way that brackets work in a mathematical expression is a closer analogy. In addition to matching opening and closing brackets, a mathematical expression can use several types of brackets. The bracketed expressions can be nested inside one another, but cannot be interleaved. This is very similar to how blocks work in a workflow.