One way to understand how Solr works is to think of a loose-leaf book of recipes. Every time you add a recipe to the book, you update the index at the back. You list each ingredient and the page number of the recipe you just added. Suppose you add one hundred recipes. Using the index, you can very quickly find all the recipes that use garbanzo beans, or artichokes, or coffee, as an ingredient. Using the index is much faster than looking through each recipe one by one. Imagine a book of one thousand recipes, or one million.