It’s the same contract you ‘sign’ with your friends or co-workers. People, especially in this thread, break it out as some solid ‘thing’, but it’s like any other ethereal concept that gets referred to by a concrete word. English is hard and not every word brings along every element in every instance. You could say that an ‘agreement’ must have a written, or at minimum a spoken set of terms, but you could have an agreement not to physically fight someone just by a few movements of your body, and ‘break’ that agreement by broadcasting one set of signals and then taking a swing at them.