Render React on the Server with renderToString //

Hello campers, I passed this challenge but I couldn’t understand what is the purpose behind this methode
although the explanation provided from FCC team but I am not able to figure out what is happening on the sever side ,and also even I can not use node.js is that okey ?
despite of searching on the intenet about renderToString() method ,I found some vidieos but they confused me even more .
here is the link :