The act of Borrowing Five Dollars is when somebody borrows 5USD or another country's monetary equivalent from somebody, usually his/her homie, or the homie of a homie. Borrowing five dollars from somebody one does not like would be difficult to do, since the borrower dislikes the lender (or vice-versa), and would not want to ask to borrow five dollars, or the lender would not give the five dollars.
It is also possible to ask to borrow more than five dollars, though borrowing five dollars is the most common amount of currency one might lend to another because it is a small enough amount to give. If the borrower and lender are tight, the lender may allow to lend a larger sum to the borrower, with the limit of lending reaching up to twenty dollars.
Dashie will regularly make jokes about borrowing five dollars, and can sometimes make his own jokey explanation for hostile relationships or natures between video game characters on his gaming channel, once claiming that the reason Bowser hates Mario is that Mario asked him for five dollars but has not paid him back yet.
The borrower is expected to pay the amount of money back later. Not doing so is fucked up. The lender might let it slide, meaning the borrower does not have to pay the lender back, or the lender has forgotten about it.
A typical act of "borrowing five dollars" might go like this:
Borrower: Hey, man.
Lender: What's up?
Borrower: Can I borrow five dollars real quick?
Lender: Yeah, sure thing.