A "glowing" letter is one that praises the candidate in the highest terms on several criteria: "One of the three best .... I've ever worked with...". The letter will detail both past performance and make positive predictions about the future.
However, it is possible for such a letter to overdo the praise and then be discounted by the reader as shallow.
What you want a letter on your behalf to say is that you are knowledgeable and that you work hard and that you have a lot of ideas and potential. A glowing letter will reinforce those ideas with examples and/or comparisons. In some fields, positive statements about teamwork can also be important. In others, being seen as self-directed is more important.
But it also needs to be realistic. "Candidate calms hurricanes and has the compassion of a saint" is probably too much to believe.