The term "Boolean" comes from Nineteenth Century mathematician George Boole. Boolean logic is used heavily in computer software, from 3D modeling applications to search engines.
There are two basic Boolean search commands supported in Google, AND and OR. AND searches search for all the search terms, "sausages AND biscuits," (all documents containing both sausages and biscuits) while OR searches search for one term or the other, "sausages OR biscuits." (all documents containing either sausages or biscuits)
Google defaults to AND searches automatically, so you don't need to type "AND" into the search engine to get that result.
If you want to find one keyword or another, use the term OR. It's important that you use all caps, or Google will ignore your request.
To find all documents containing either sausages or biscuits, type: sausages OR biscuits. You can also substitute the | character for OR, so sausages | biscuits searches for the same thing.
If you're searching for a phrase rather than just a single word, you can group the words together with quotation marks. Searching for "sausage biscuits" will search for only the exact phrase sausage biscuits. It will ignore sausage and cheese biscuits. Searching for "sausage biscuits" |"cheese sauce" searches for either the exact phrase sausage biscuits or the exact phrase cheese sauce.
If you're searching for more than one phrase or keyword in addition to the Boolean, you can group them with parenthesis, such as recipes gravy (sausage | biscuit) to search for gravy recipes for either sausages or biscuits. You could even combine exact phrases and search for "sausage biscuit" (recipe | review).