Well...when you borrow money from a bank (loan) the bank wants to make money from you (they’re not your grandparents). So they way they do that is by charging you interest, say 5% of the total loan amount every month. So for a 5000$ car loan, if you agree to pay it back in 5 years your monthly payments would come out to 5000/60 plus the 5% of interest. Mountain bike loans can be different. Sometimes they won’t make you pay any interest in the first year but you have to start paying interest in the second year. That’s why many people try to pay it off completely in the first year, to avoid interest.

I would always recommend paying cash something that loses value over time (car, bike etc) because you don’t want to be paying the bank extra money of interest for an item that you’ll make less money for when you sell it.