What does the E mean after my carrier on the iPhone?

I have T-Mobile and once in a while a large E pops up after the carrier on my iPhone… What does it mean?

It is nothing to be concerned about.

Usually, you will see the type of signal next to the carrier. It can be LTE, 4G (HSPA+), 3G, and E. E means EVDO which is pretty much 2G, 2.5G. EVDO is very slow, don't expect internet to be on your iPhone with the letter E next to it. Other then that, nothing bad. Its just the type of signal on your device.