To indent is to begin text with a blank space between it and the margin. When you're writing an essay, you can indent the first sentence of each paragraph.
There are different styles of arranging type on a page, but it's fairly common to indent the beginning of a paragraph or section, which you can do with the tab key on a keyboard. Another way to indent is to cause a hollow, depression, or notch — in other words, to dent. This meaning is closest to the original, "to notch or give a serrated edge to," from Medieval Latin, indentare, "furnish with teeth."