Ovid displays the results for each of your queries in numbered sets, which have no relation to each other until they are combined. Combine sets by selecting the ones you wish to join and clicking AND or OR. Although synonyms are normally combined with OR, here's is a repetitive example of AND, which shows how many citations contain both a keyword phrase and its related MeSH.
Compare the results for the above sets to see the advantage of using the exploded MeSH. Ovid automatically explodes selected MeSH to include more specific MeSH in its tree.
Additionally, if you creat an account in Ovid, you can Save Search History. This can be helpful if you are doing research over a period time, because it allows stores searches you have already done letting you run it at another time to check for possibly new finds.
Limits restrict search results to selected criteria. The most commonly used limits are listed in this blue area on Ovid's main search page. After checking one or more, click Search to restrict the results of your last set. To limit an earlier set, enter the set number and do likewise.