Questioning is a term used to describe anyone who is in the process of deciding which gender identity suits them best. Transgender people may undergo a period of questioning in which they reflect upon their preferred gender expression, their personal definitions of masculinity and femininity, and their feelings about their assigned sex.
Because gender identity is an internal experience, no-one else can know for certain someone's gender or "answer the question" for them, although learning more about the experience of others can help with self-understanding. However, a period of questioning only ends when the person decides for themselves which gender identity, if any, they feel most comfortable with.
Questioning people with supportive friends may undergo trial periods of social transition, in which they ask a small group of people to use a different name and set of pronouns in order to experiment with which gendered language they feel most confortable with.
A period of questioning can occur at any time, as gender identity can sometimes change without an obvious external reason. Most questioning people previously considered themselves cisgender, but some transgender people may begin questioning their previous identity and adopt a new one.
Questioning should only be used to describe those who are seeking to label their gender identity, not those who are comfortable without adopting a label.