Your browser stores recently visited webpages as temporary files in a cache, which enables the pages to load more quickly when revisited.
However, a full or corrupted cache can cause problems in TurboTax, so sometimes you need to clear your cache (that is, remove these temporary files).
Here are the official instructions for the most popular browsers:
Apple's official support site doesn't appear to contain instructions for clearing the cache in Safari. Your best bet is to search the Internet for clear Safari cache or similar search terms.