Expanding and minimizing
You simply start by typing in one search term, chances are that you will get too many results of which many are not relevant. By adding a second search term, you already search more specifically. Both one and the other search term must occur in the found websites or documents. You will now have fewer results but more specific results. If you are still not satisfied with your results, add another search term. Keep doing this until you have a useful list of results that really deal with your topic.
In the following example, one search term is added at a time. You will keep on combining until the result consists of enough relevant hits.
In PubMed we will start with the search term children. We have added the search term sleep, and finally the search term bed. As you can see the number of hits will decrease.
Step 1: searching with one search term: children
Step 2: Adding a second search term to specify: sleep
Step 3: adding a third search term to be even more specific: bed