It is very tempting to just put a tick in the box next to a suitable MeSH and then click the Continue button when doing initial searching in Medline (Ovid) BUT be very careful as some MeSH may not cover what you expect.
Though pedantic it is important to click on a MeSH (in the following example Depressive disorder) to see its place in the tree. Medline (Ovid) even instructs you to 'click on a subject heading to view more general and more specific terms within the tree'. In the tree structure you can see there are specific types of depressive disorders and you need to decide if you want to include all of these as part of your search, a couple of them or only one.
Click on the i in the Scope column for a heading to see a definition and coverage. The Scope note for Depressive disorder for example indicates there is also a MeSH Depression and there is a difference between depressive disorder and depression. You would need to read the scope note for depression to decide which is the MeSH you want to use.