I first heard the term "stretch assignment" in undergrad from a lecturer discussing career development. She encouraged us to identify those opportunities that are a bit beyond our current skill set and comfort zone, and embrace them as an opportunity for growth.
Did you know that women are more likely to apply for a job only if they meet all of the listed qualifications? But men will apply even if that last few bullet points don't describe them. Once I read this statistic, my entire approach to growth transformed. Many of my peers, upon graduation with their Master degree, are job searching. For myself, I am preparing myself for doctoral training, and reflecting back upon these past 2 years. What projects would I like to spend my time on? How will it influence my training and career path?
Whether you are starting a new job, or you are joining a research group, communicate your goals clearly to your advisor. What do you want? What skills do you want to add to your resume, and how can you do that? Tell your superior you'd like to gain experience writing and giving presentations, or improve your statistical skills. They'll keep you in mind as projects come across their desk.
For example, my goal over the next academic year is to focus on publishing. Communicating this with my advisor means they will not only recommend me as a first or junior author for colleagues, but they can better guide me through the maze that is academic publishing.
Learn by doing!