Answer 1

its linking external libraries for you to use. much like you can link your own javascript file in your html..you can also link others.
its called cdn..content delivery network.
popular ones are google fonts..bootstrap..jquery..ect.
if you dont want to link them..you can install them locally in your project..but that cant be done on sololearn so cdn is the way to go

Answer 2

Jason is right, I just want to add that even though outside of sololearn you could download it and serve it yourself, CDNs are usually better for popular libraries because they are already loaded in users' cache. This means that for a popular library like jQuery, if all sites were to use the same CDN, the downloaded file would be cached and reused instead of redownloaded, which can improve page load times significantly on slower networks since some libraries are pretty large. CDNs are also usually spread out on servers in datacenters around the world and very optimized for what they do, so even loading a resource from nothing is probably faster from a CDN rather than your own server.