The idea is that gender identity is something you’re born with but gender expression is a choice. Consider three examples:

1. A trans person in a community that doesn’t accept them may decide to dress and act in accordance with their biological sex even though (secretly, on the inside) they have a different gender identity.
2. Tomboys. Born as a biological female, identifies as a woman, but she expresses her gender in a way that people usually think of as being masculine.
3. Drag/Crossdressing. You can be born as a man, identify as a man, but enjoy wearing woman’s clothes (as part of a performance on stage or just because you like it).

Just like sex and gender are separable, gender identity and gender expression are separable.