Mobile friendly is a phrase that means a webpage is suitably rendered on all screen sizes. In early 2016, Google announced what became known as "Mobilegeddon", that penalties would be dished out to sites which had pages that were not mobile friendly. It was a bit of damp squib but mobile friendliness is certainly taken into account by all search engine algorithms as it's so important for good user experience.
You can check manually if a webpage is mobile friendly by reducing the window size of your browser and having a look for yourself how the page renders at different screen sizes. Alternatively, you can use Google's Mobile-Friendly Test tool or Bing's Mobile Friendliness Test tool.