How do I "Parallelize downloads across hostnames"? Please help!

[Waves and Willows]

Hi,

My WordPress site has terrible performance even though I've tried to follow all of the best practices. The worst result I got (and there are quite a few bad ones) was an "F" for "Parallelize downloads across hostnames".  I just don't know what to do about this and with my lack of experience with anything like this, I don't want to do anything that will mess up my site.  Does anyone have any advice for me?

Thank you!

Jennifer