What does LS (least square) means refer to?

I have been reading clinical papers and recently come across the term "LS-means", referring to what seems to me as an estimation of some population's mean measure. Obviously, I know what "mean" refers to and I know when one estimates a mean for a population from a sample, one has to put some measure of confidence to it, or a measure of standard error, otherwise it's just a number - this does not seem to be the case with LS-means measure (at least not in the papers I encountered, maybe they just did a sloppy job, I don't have enough knowledge to tell). I also know what "least square" refers to when it comes to regression models or optimization problems.

I have never encountered the combination "LS-mean". I admit that my background in statistics may be lacking since it is not my primary field of occupation. A quick search of online sources doesn't seem to yield a satisfactory explanation of what does this combination of words actually refers to (regression? estimation?) and how does least-squares method (of what I can only assume is optimization) fit with calculating an average measure of a population.

Example of something one may find in clinical literature (paraphrasing): A clinical trial of a drug was conducted against a matched placebo control group. This trial lasted several weeks. Each week the subjects were tested for symptom severity. The results of the study were presented in a chart "improvement from baseline vs week of treatment". X axis - weeks, Y axis "LS-means change". Change in placebo group was significantly smaller than drug group - hurrah our drug great.

I know that this question is very broad, so to limit the discussion, these are the things I am looking to find out:

(1) Can anyone tell me what "LS-mean" may be referring to in the context of clinical trials (or any experimental work for that matter). It doesn't have to be long and/or exhaustive - a few bullet points with keywords would be great to start my self-education journey.

(2) A good online source or a book for getting up to speed on the topic of "LS-means", whatever it may be referring to.