Leading spaces are dropped from normal paragraphs, and four of them (or a tab) turn the line into a code block. To indent prose, use space entities such as   or . To indent a block of content, use a blockquote or a nested list.
All ways to write it
An em space entity. Use two or more for a deeper indent.Works everywhere Indented paragraph
Four non-breaking spaces.Works everywhere Indented
A blockquote indents a whole block, with a bar beside it in most themes.Works everywhere> Indented block
Example
You type
Normal paragraph.
  Indented with two em spaces.
> Indented as a quote block.You get
Normal paragraph.
Indented with two em spaces.
Indented as a quote block.
Questions
Why did my indented text turn into code?
Four leading spaces or one tab make an indented code block. Remove the spaces, or use   for a visual indent.
How do I indent a list item's paragraph?
Indent the continuation line to line up with the item's text (two spaces after "- ", three after "1. ") and leave a blank line before it.
Indent in Confluence
Confluence's editor has indent buttons for paragraphs and lists, so on a page you don't need entities. Inside a Markdown Macro+ macro, the rules above apply, and the preview shows when four spaces have turned text into code.