A mask attack is a targeted form of brute force attacks in which the attacker exploits patterns in known passwords to reduce the number of attempts required to crack a password.
An example of this is as follows:
An attacker knows that their target is using a specific make and model of wifi router. Looking at a few different units of this make and model, the default password is always using the pattern of <adjective><noun><three digits>. The attacker can build a custom word list to match this pattern.
Another example is the same scenario as above, but the passwords are more randomized. After observing a handful of models, the default passwords are always an upper case letter, two digits, three lower case letters, a number, and an uppercase letter. The attacker can create wordlists or use a mask format reflecting this pattern in their password cracking software to only try passwords matching this pattern, reducing the number of attempts required to crack the password.