Keywords are the central concepts or ideas that you are searching for--they could be people (Calico Jack, pirates), places (the south sea, Bermuda), or things (the jolly roger, astrolabes). But, they may also be concepts or ideas (egalitarianism, mercantilism).
One of the things to be aware of in keyword searching for primary sources is that your keywords need to match those in the primary sources in order to find the best results. Sometimes that may be a matter of finding the comparable term for the period you are interested in. Almost always, it means you should consider a list of synonyms for your keywords (pirate, buccaneer, sea rover, privateer). But, with primary sources, especially those from the 18th century or earlier, it also means contending with variation in spelling.
A researcher interested in hurricanes in the early Carribbean would need to not only use "hurricane" as a keyword but consider synonyms like whirlwind, tempest, storm, and squall. Furthermore, they would need to search for hurricane, huracan, furcanes, hurry-cano, or hurricano. Likewise, for whirlwind, they might need to search for whirlewind.
Accounting for variation in words and spelling from the start will help you find the sources you need. The tips below can help facilitate the process once you have identified your keywords and search terms.
Tips for Multiple Search Terms
Utilizing a phrase search assures that your results prioritize or are limited to sources that use the two or more terms you search together as a phrase the way you intend, rather than finding results that have each of the words, but not necessarily together.
To search a phrase, place "quotation marks" around your terms.
Example: "jolly roger" will get you results about pirate flags but jolly roger without quotations will deliver you sources like the book Uncle Wiggly's Adventures, which happens to have the name Roger, and the term jolly in completely distinct parts of the book (which has nothing to do with pirate flags).
Inserting a wildcard in your search helps to search for variations, and can be especially helpful when searching for primary sources that use a long s or to account for non-standardized spelling.
To use a wildcard, insert at question mark (?) in place of the letter or character you think could vary.
Example: p?rate will find both pirate and pyrate
Truncating your keyword allows you to search multiple variations of a word at once based on a shared stem or root.
To use truncation, insert at asterisk (*) at the end of your root or stem.
Example: buccaneer* will return results for buccaneer, buccaneers, and buccaneering
Example: mutin* will give results for mutinous, mutiny, mutinies, mutineer, mutinying, and mutinied