Websites store data (like images or bits of code) in your browser's cache to speed up your browsing experience. Your cache is updated when new versions of that data is detected, but occasionally issues can occur when there is a mismatch between the latest version and what's stored in your cache.
A hard refresh reloads the full website discarding anything stored in cache.
Most browsers have an icon like this you can click to perform a hard refresh: 
You can also use these keyboard shortcuts for your particular OS and browser.
Mac
Safari- Command+Option+E then reload the page
Chrome- Command+Shift+R
Windows
Edge- Ctrl+F5 or Ctrl+Shift+R
Chrome- Ctrl+Shift+R