It means to conform to the image/stereotype that exists in society about how an ideal male should be and how he should behave in certain situations. Just like being feminine means to conform to the image of an ideal female, too, and behaving as such.

There is no difference. The only difference is that the modern society, with the help of the brainwashing that is called “feminism”, has convinced most people that femininity is mostly good and masculinity is mostly bad. There is no difference, both are based on a set of ideas of how you should be and behave. Both are toxic, not just masculinity, because all ideas about how you should be are toxic, and they rid you of the pleasures of life.

Don’t be masculine, be yourself!