If you look around - you'll see tennis elbow braces that cost (not for two - just for one) $45, even $100, and I'm sure they work great.
What's even better with this pair of braces, is that they work great too, but only cost $10 for one (the you're getting two braces here for $20)!!!
If you play guitar, then you probably know what a "Capo" is, and if you don't - you put a capo around the neck of your guitar, and it has a metal rod covered with a soft pad usually made of PVC, which, when you tighten the capo around the guitar neck, allows you to effectively make the whole six or twelve strings (etc for bass - same concept) shorter, thus raising the pitch.
This is what an elbow brace does - they all work on the same principle, only the purpose here is to isolate and reduce the load and vibrations from the hurting part of the muscle and tendon, as opposed to shorten strings on a guitar.
This brace works as advertised and as it is supposed to - assuming you understand what to expect from it and use it correctly, but costs a fraction of what many other braces out there cost.
You should also realize that if your elbow tendon and muscles hurt so much that a brace like this couldn't make it better over a few of days - there's a good chance that either you don't have Tennis Rlbow or Golfer's Elbow but maybe something else, or that your muscles or tendon inflammation are so bad, that you should probably see a doctor, as a brace like this may not be the solution to what you might have.