The Grumpy Cat mask is a costume item representing Tardar Sauce, better known as Grumpy Cat, an Internet celebrity popularized because of an underbite and feline dwarfism giving her a permanent frown. An officially licensed version of this has been created, and is made of latex covered in faux fur. The wearer sees out of slits around the mask's eyes.
Although a full costume has not been officially licensed, the wearer can obtain a mascot outfit, or simply wear a shirt with a Grumpy Cat meme on it. Even without a further costume, this character is iconic and the wearer should be widely recognized.
The wearer may also wish to act "in character" to help the costume go over even better. Getting into character as Grumpy Cat is easy, and just involves being dramatically unenthusiastic or unamused by various things -- even so much as to anything and everything. Be careful not to annoy other people too badly in the process, however.