Creating indents in a word processor is a trivial task that you hardly even need to think about. In HTML-based formats indents are not quite so straightforward. HTML does not support either tab stops or multiple spaces, so you cannot use them to create indented paragraphs or to format tabular data. (Tabs and multiple spaces wouldn't work in HTML because the page width is dynamic.)
You can use indented paragraphs – Help+Manual will convert them to stable HTML structures when you publish to HTML-based formats. However, you should avoid using tab stops and spaces on their own to create indented effects, because these will not work in HTML-based outputs. You should always use the paragraph indenting functions described below.
For some more background information see Tabs, indents and HTML in the Reference section.
You cannot use the indent tools on single-level bulleted and numbered lists. To change the indent in a list you must first switch off the list with the list tool. In outline numbered lists the indent tools change the list level.