What is a list semanitcs in python?
The term semantics is sometimes used in a programming context to describe the set of behaviors an object has. So in this case, as Zero mentioned in his comment, it means "has the same behaviors as a list".
You could alternately interpret the phrase as "is a list-like thing" or "is interacted with the same way you would a list object".
The term semantics means "meaning". In the case of an object, its meaning comprises "what you can do with it" and how. In this case you can do "list-like" things with it, like iterate its members with for x in xs, get its length with len(xs), and access elements by index with xs[i].
In context of the following para :
prs.slide_layouts is the collection of slide layouts contained in the presentation and has list semantics,
at least for item access which is about all you can do with that collection at the moment.