Basically, the busy beaver function tells us the maximum number of steps that a Turing machine with a given number of states and symbols can run for. If we know the busy beaver of, for example, 5 states and 5 symbols, then we can tell you if any 5 state 5 symbol Turing machine will eventually halt.

However, you can see why it's impossible to in general find the busy beaver function- you'd have to know which Turing machines of a given size halted, which is in general impossible.