What is an indentation?
It usually means that the line is indented... meaning, simply,
that it is moved to the right. This line is indented, for instance.
You indent block quotations usually, and the first lines of
paragraphs in papers. Word processing programs allow indentions
using the Tab key, and block indentions usually with the ruler bar
or something similar, so that the margin is actually moved and you
don't have to indent each line separately.