Most of the time, a comma won't follow the last item in a list.What is meant?

Answer 1

I suppose you are talking about Python. A comma after the last item in a list, tuple or dictionary is no problem (in fact, most languages are okay with it). Moreover, a comma in the end is used to indicate a tuple if there is only 1 item in the tuple. For example
type((1)) will return int
type((1,)) will return tuple

Answer 2

It can be convenient to have a comma in the end, if you expect to extend your list later.

data = [
item1,
item2,
item3,
]

Leaving the comma, you prevent forgetting it later, when you add item4 (getting strange errors).