The concept of indenting is quite common in writing. Whatever the left margin happens to be, the indented text begins to be displayed several blank spaces from the margin edge. This serves to "set apart" this beginning point from the rest of the text that will be aligned at the margin. (The first word of this paragraph is "indented" a few spaces.)
In an Excel cell (bet you can't say that 5 times quickly!), you also may have the need to indent. Let's take a look at what an indent will look like in Excel.
In this example, the cells are displayed with normal (no indent), one indent, and two indents. Depending on the width of the column, you could have more than two indents. Also note the "size" of the indent depends on the font size you have chosen for that cell. Excel provides two buttons (Increase and Decrease) for adding/removing indents. If you hover your mouse over the buttons you will quickly identify which button is which.
One very good question should be asked here..."When would I ever need that feature?" We haven't a good answer because the need depends on your Excel project. Perhaps this example will give you some ideas where you might use indents.
Consider what spacing you might want if you decided to put an outline (I, II, III, ... with A, B, C, and 1, 2, 3, ..) in an Excel project. You might not want to use different columns to display the spacing requirements for your outline. By proper use of the indent feature, you could have All of your text in a single column.