A concordance is a tool that scholars used in the past the same way we use a search engine today.
The most popular concordance is the Strong’s concordance. It catalogs every word in the Bible, giving each one a number, and then lists each place in the Bible where each word is used. If you want to study a word, such as love, you would look up the word love in the concordance, and it would give you a list of all the verses that use that word. It’s basically the paper form of a search engine.
You can still use a Strong’s concordance to great effect today, but you can also use a website like blueletterbible.org to achieve the same thing. - By the way, they named it blue letter bible because hyperlinks on web pages are colored blue, and they created this site to have a link to every word in the Bible.