Diastasis Recti by definition is a gap between the right and left side of your most superficial ab muscle, or the rectus abdominis. This is super common following pregnancy, but can occur in both men and women, and is caused by excessive and unsupported intrabdominal pressure. A quick way to diagnose yourself is to lay on the floor in sit-up position and slowly lift your shoulder blades off the floor towards your knees; if your belly comes up towards the ceiling instead of contracting down towards the floor in doing so, then you probably have some abdominal separation.
This is a problem because everything stems from your core; you need it for important things like keeping you upright, stability, and looking good in a bathing suit. (;
What you can do to fix it involves both A) fixing your body’s alignment, addressing imbalances you may have, and B) strengthening the other abdominal muscles around your rectus abdominis, including the transverse abdominis, which runs from your spine all the way around your stomach, and the pelvic floor, or deeper ab muscles.
For alignment, foam rolling techniques, I really like Muscle Whisperer Josh.
For exercises to address this issue, I really like Dr. Jared