oh god, you don't even know how to google. How do you get your information? How do you learn things? Do you just listen to whatever people tell you and that's it?
A "block" is a city block. Most cities are based off grids so many streets intersect with each other, However, if you're in the suburbs, it is used there as well because streets still intersect with each other.
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Each intersection of lines here is a block. Let's say that someone says to go four blocks down the road. Therefore, I go past four street intersections.
That poster saying "3 blocks down" was using that phrasing to denote that the thread you were looking for was in another place and you're in the wrong place. It was an analogy. Hopefully you know what an analogy is.
Is English your second language? Do you live in a rural area? I don't know anyone who speaks English as their first language that doesn't know what a block was.