Ant-Man and the Wasp set up an entirely new - and as yet unexplained - powerset for Janet Van Dyne. The Quantum Realm became crucial to the overarching narrative of the MCU's Phase 3 after it was first introduced in 2015's Ant-Man, where it was described by Hank Pym as "a reality where all concepts of time and space become irrelevant as you shrink for all eternity."
Decades earlier, Hank had lost his beloved wife Janet Van Dyne to the Quantum Realm. He believed it was impossible to rescue her - and was proved wrong when Scott Lang successfully managed to escape. That directly set up Ant-Man & the Wasp, in which Hank successfully invented a Quantum Pod to safely enter and exit the Quantum Realm. This was a tremendous accomplishment, suggesting Hank Pym is actually the greatest genius of the MCU; Marvel has recently published an in-universe reference book called The Wakanda Files, in which Shuri expresses amazement at Pym's designs. To Hank's delight, he was successful, and rescued Janet.
But Janet Van Dyne was subtly changed - and according to an official Marvel book, The Wakanda Files, it's a permanent change. The book includes a handwritten note from Janet discussing her new powers.
"I have quite a bit to catch up on. The world is a changed place. Then again, so am I. Part of it is adaptation, part of it is evolution. I'm suddenly capable of the previously incapable. I'm stricken with a form of Quantum Entanglement, as if every molecule in my body continues to be in multiple places at once. I believe that's how I was able to feel [Ghost's] pain. I'm not entirely sure how, but I was able to phase her complately back into our reality. Perhaps there are healing qualities to the particles in the Quantum Realm? It feels like I was in there for a lifetime, but I'm left with several lifetimes worth of questions."
Time doesn't operate normally in the Quantum Realm, so it's actually impossible to estimate just how long Janet was in there in terms of her own experience. What's more, it's possible even the briefest amount of exposure to the Quantum Realm has the potential to grant these abilities; Scott Lang was able to break out of the Quantum Realm because he sensed his daughter Cassie calling out to him, and that sounds similar to the Quantum Entanglement described by Janet. But Janet's abilities lasted even when she had left the Quantum Realm, and she clearly retained other powers as well.
The most intriguing comment, however, is that Janet Van Dyne feels as though "every molecule in [her] body continues to be in multiple places at once." This may suggest that, like Ghost, she isn't truly in phase with this dimension at all, that her molecules are phasing between this plane of existence and other realities. If so, Janet Van Dyne could be a key player in the MCU's Phase 4 and 5 slate, which appears to be building on the concept of the Multiverse. The first two Ant-Man films were crucial in establishing the MCU's overarching narrative, and the same could be true of Ant-Man 3 as well.