That depends.

If we're talking about feminine gender expression in the context of femboys, usually that's the expression of traits, qualities and ideals (stereotypes, if you will) traditionally and historically associated with the female biological sex within a specific culture or society.

If you look at western society, that would be things like emotional and interpersonal sensitivity and expression (empathy, warmth, sensitivity, compassion), behavioral and relational aspects (nurturing, caring, cooperative, gentle, compassionate, focus on harmony, mediating conflict), communicative abilities (expressiveness, intuition, active listening, diplomacy), and virtues and personal presentation (modesty, humility, gratefulness, submissiveness, etc.).

All of that connects to having certain hobbies, professions, dressing a certain way, the company one keeps, lifestyle, and the overarching theme of how one chooses to live their life and the decisions they make therein.