Savitar's weakness was finally exposed on The Flash thanks to some quick thinking from H.R. and Jesse. After Savitar tricked Wally into taking his place in a Speed Force prison, Jesse was anxious to beat him once and for all in "Into the Speed Force." So, while Barry journeyed to the Speed Force to find Wally, Jesse went after Savitar by herself, and what she found could be a way to defeat him once and for all. Savitar is human on The Flash — he's not a speed god, he's just a man.
Fans have always suspected that Savitar was a human, perhaps even a character we've seen before, but this week it was finally confirmed when Jesse convinced Cisco to run some tests on the chunk of Savitar's armor. Together, they realized that Savitar's armor was vibrating, trying to get back to the rest of it, meaning it could lead them directly to Savitar. In a foolish act that everyone was convinced would get her killed, Jesse took the armor and used it to track down Savitar. Luckily, during the fight H.R. connected to her through coms and realized that his armor must be protecting him from something. If she could find the weak spot in his armor, she could potentially get an upper hand. And, that's exactly what she did, stabbing the piece of armor directly into his shoulder.
Savitar let her go and sped off, confirming that beneath his armor, he's just as vulnerable as the rest of us. So, what does this mean for Barry and the rest of Team Flash? Well, for one thing, it means he's very possible to defeat. Savitar isn't a god of mystical energy, he's a man, which means he can die. (Though, considering the fact that a speedster has to take Jay's place in the Speed Force prison, I'm guessing Barry isn't going to be killing Savitar.) His weakness also means that Barry can potentially overpower him physically. There's just one thing he needs to get past first, and that's Savitar's armor. Unfortunately, Jesse basically gave the only piece of armor they had back to the big bad, which means Cisco and Caitlin won't be able to analyze it any more than they already have.
Still, the discovery of Savitar's weakness is huge, and hopefully it will help Barry defeat Savitar before he kills Iris, something the Flash has to stop at all costs.