wp bcrypt switches WordPress's password hashes from MD5 to bcrypt, making it harder for them to be brute-forced if they are leaked.
WordPress uses phpass to store passwords. Because WordPress has to work everywere, it uses the portable version of phpass, which uses MD5 to hash passwords. MD5 is not a very good hashing algorithm for passwords, because it's relatively fast.
This plugin switches over to bcrypt, which is the algorithm recommended by phpass, and is a much better option for password storage because it is much slower to produce. This makes it much harder for an attacker who's managed to access your hashed passwords to obtain plain text passwords by brute-forcing, or by trying passwords from a dictionary.
Note: this plugin requires PHP 5.3.0 or newer
Be aware that if you use this plugin and then move to a host that does not support bcrypt, you will need to reset any user account that you want to log in with.