While I’m not an expert by any means, I look at it similarly to how I look at mental health in the workplace. You simply aren’t going to get the best results from people until you recognize that they are people first, and employees (or students) second. That means that they cannot perform their best when they are also dealing with various life situations and struggles that have nothing to do with the immediate work at hand, and we would get better results if we made efforts to support the whole person and not just the robot that is there to do work.
SEL is that for schools. It recognizes that kids come to school with a variety of issues that would hinder their ability to be successful in school, and makes an effort to support them in those struggles so that they can be more successful not only with school work but with the interactions they have with teachers and other students.
But, this acronym has gotten mixed in with all the other things that some parents are upset about and they are demanding that schools stick to teaching math, science, and reading instead. Let me try and point out why this is a mistake.