Have you ever been to an all-you-can-eat-for-a-fiver pizza buffet?
Imagine there is one buffet cart constantly topped up with pizzas; this represents available bandwidth. Now imagine the restaurant can seat up to 50 people; this represents broadband users.
The contention ratio is therefore 50 people to one buffet cart.
The restaurant's manager is confident that
even when the restaurant is full (50 diners) the buffet cart
will have enough pizza for every guest to have a fair and
belly-filling amount. This is because people usually take
one or two slices at a time.
However, if some diners in the restaurant are feeling particularly hungry, they may be tempted to take several pizzas at once, leaving none for the other diners.
The buffet can therefore only work as intended if all diners appreciate it's a 'contended' service.