Ones gender identity is simply the gender that one is. Ones gender is decided by and for ones self without regard to what one was assigned at birth or what other peoples opinions of gender are.
The importance of gender (identity) for someone can range from being very important to being something that one is indifferent about.
Someone's gender (identity) is generally what one would answer to the question "what gender are you?" - such questions are often asked online, where one may have a username and picture on social media that doesn't doesn't indicate a particular gender to the person asking the question.
In addition, the attached Venn diagram may help in answering this question. where the five parts (assigned gender at birth, gender identity, gender expression/presentation, gender role and, romantic or sexual orientation) are things that do not overlap, although for some people it maybe that these things are apart of how and why they identify the way they do, but that is up to each individual to decide for themself.
For example, many people identify with the gender that they were assigned at birth, and some do not. Furthermore, someone may identify with being a man while expressing his gender in a way that's different to what is typically expected of men in his specific culture.
There are many myths about gender that can make this topic hard to understand, as many people believe that certain things outside their decision making dictate what gender they are and therefore the genders other people.
However, with the way gender is described above, an individual can decide what they believe without having to dictate this for others, which allows for people to freely be who they are and talk about what gender means to them without having to tell others who they are and what to do.
