Can-do is an adjective and we often see it combined with attitude. A person with a can-do (or can do) attitude is one who remains positive in the face of challenges, one who believes that things can be done with effort, and that obstacles can be overcome. This is the kind of person who responds positively to ideas and makes things happen. A can-do person is the opposite of a naysayer. "Nay" is an archaic form of "no," so this word means a person who says no to things routinely, even things that have a lot of potential. In other words, a naysayer is generally negative about new ideas, while a can-do person is generally positive. As you might expect, a can-do attitude is considered valuable in professional life, and some job descriptions include it explicitly.
The image here is of the famous Rosie the Riveter, an iconic figure from WWII America.