Certain kinds of sources will be particularly useful in the early stages of a research project. Often called "research starters" or passages from reference books, (which can be in print or digital) these sources offer encyclopedia-like entries on your topic.
Whether you are new to the topic, or just trying to confirm what you already know, having access to a subject specific entry will make more advanced searching easier. These sources tend to be brief summaries of what experts consider to be the most important things to know about the topic.
Example: Allied Health