In order for me to explain this, i will correlate it to the experience we might already have such as a zip file with password. In this case, if you want to open that zip file and you didn't know what the password is, you can use the method called "Brute Forcing". Wherein, you will use a software that will try all the possible password based on the number of characters you set, and if the password matches, you could open the zip file. The same with bitcoin private key, the reason why it is too hard for us to brute force private keys is because of its number of characters, the possible combination of characters is too difficult to brute force as it takes too much time, plus, if the system uses strict security features, most of the time, they limit the password input to 5 times, then your account could be locked. But if not, you'll need a very fast computing system to conduct this procedure.
These aren't quite the same. To brute force a password is dependant on how long the password is and what characters or symbols it uses. If the password is a word from the dictionary it would be near instant to brute for it. But if the password is some 20 characters, numbers, symbols long then it would take a very long time, if ever to find the password. There is no password limit really. This isn't some exchange.
Bitcoin Addresses are difficult to brute force because the private key is very long, and there was a thread in this section were someone started to brute force them from private key "0" and they only found addresses containing a balance because people purposely sent it there as a contest but a real random generated public key hasn't been found yet, at least with a balace or even with a past transaction.
So BTC addresses are very safe from brute force at the moment.