This shows the position of the first line of a paragraph, or, in a bulleted or numbered list, the position of the bullet or number for an item in the list.
Left indent markerThis shows the position of the second line of a paragraph, or for a bulleted or numbered list, the indent of the text next to the bullet or number.
Default tab stopsThese appear as small gray markers underneath the numbers on the ruler. You can drag these to new positions. Use them for text that's not in a bulleted or numbered list.
Customized tab stops Further customize your tab stops by setting left 
, right 
, center 
, or decimal 
tab stops.
A typical example of changing an indent is when you want to adjust the space between a bullet in a list and the text it goes with