Wordle is a tool that generates word clouds from input text where more frequent words appear larger. Users can go to wordle.net, paste in text, and click "Go" to generate a word cloud image that can be used to introduce topics, analyze text content, compare texts, and more as suggested in the document and blog post provided. The document provides instructions on how to make a word cloud using Wordle and gives examples of how the clouds can be used for teaching and learning.