STEM stands for science, technology, engineering, and math. Women in STEM refers to women in these fields. This phrase comes up to represent women in traditionally male-dominated roles.
Feeling like you don't belong where you are or don't deserve it can be a huge hurdle to overcome for a woman in the workplace. That is especially true for women in STEM, a typically male-dominated field.
This is a big topic nowadays, especially when considering gender equality. It affects everyone, but women are especially susceptible to the adverse effects of lack of transparency.
Even if the mentors are there, women in STEM often don't feel they have access to them. The reasons for this vary depending on the job, industry, and the people involved.