According to Apple’s support page, iOS 9.3.4 will fix “A memory corruption issue was addressed through improved memory handling.” If you have jailbroken your device, safe to say that updating to iOS 9.3.4 will no doubt revoke jailbreak privileges, but if you don’t jailbreak your iOS devices, or if you feel that maybe your device’s security is more important than having it jailbroken, then we guess you should probably download it.
It has been speculated by MacRumors that iOS 9.3.4 will most likely be the last update to iOS 9 ahead of the release of iOS 10, which last we checked was still in beta. However Apple does plan to release it later this fall, presumably alongside the new iPhones, meaning that from now until then unless something serious pops up, there should not be anymore iOS updates.
The update is pretty small so downloading and updating should be pretty fast, but just to be safe, do connect your device to a charger.