We encourage you to tag replies that solve your questions. It helps the site in a variety of ways. For example, users will see the solution just below the question, allowing them to avoid hunting the answer in the entire discussion – useful in long discussions.
Only a few channels have the solution check box enabled. For example, the E.R, which is the forum to post questions, has it. However, if your post was created in another forum, let's say the Floating topics, and it was later moved by you or by a mod to the ER, the check box won't be visible.
Normally, the mod should have activated it (click the wrench, then "rebuild html") but it happens that we forget. So if you ever want to mark a post as a solution and you do not see the check box, click on the wrench below that post and select "rebuild html". The box will appear (as long as it is a channel with solution check box enabled) and you can click it to mark the post as the answer to the question.
Sometimes, the solution is a compound of various elements contributed by many people over many posts. You can mark the most useful one, or you can add a reply in which you summarize the many solutions and mark that reply as the solution.