Leverage browser caching means you should take advantage (leverage) of the functionality of the browser caching function. By default, browsers support caching of files until they "expire" i.e. they are deemed as stale and require a refresh. The webserver can instruct the browser how long before file files expire (for example, 1 month). By doing this, the browser can reuse the files without having to download them again, making your website load faster in the process.