NeonSturm has detailed it pretty exact mate.
'.rar' files are like '.zip' files, they are files containing compressed files.
There is a multitude of unpacking/extracting software available for free, my personal favorite being 7Zip, though WinRar works perfectly. Go on Google, find these products and download one you find suitable and free.
Once you have installed your unpacking software, you should be able to right-click on a .rar file and 'extract' the content of the rar files to another directory, such as your desktop. Extract those files to your StarMade/Blueprints directory and you should be good to go.
Best of luck!
Note: I've assumed you're a Windows user.
~CR~