Masculinity is an expression of male empowerment. What "feminism" is for women, "masculinity" is for men. And like any other gender identity, its expression is unique and deeply personal.

For a man, being "secure in your masculinity", basically just means being comfortable and confident with who they are. Without being secure in that sense of self, it's very easy to fall into more toxic ways of expressing themselves. They will overcompensate for their lack of confidence and lash out in an effort to gain control, without realizing they're trying to control things they have no real power over. And when it inevitably doesn't work, they will become frustrated and lash out even more.

This is why all these "manosphere" assholes are such terrible role models for young boys to have. The way they "teach boys to be men", is focused on gaining external control over people and/or situations...which is the most toxic way of expressing their masculinity. They should be focused on self-control and inner strength, rather than trying to control or dominate everything around them.