You are writing, and you suddenly realize you have to change sentences in a previous section to support what you are writing in this section. The problem is that you have to remember where you are right now so that you can come back to it later. Can't Vim remember it for me? This can be achieved using marks.
You can create a mark by pressing m followed by the name of the mark which is a single character from a-zA-Z. For example, pressing ma creates the mark called 'a'.
Pressing 'a returns the cursor to line of the mark. Pressing `a will take you to the exact line and column of the mark.
The best part is that you can jump to this position using these marks any time thereafter.
See :help mark-motions for more details.