Unfortunately, Riot hasn’t announced exactly what Zeri’s abilities do just yet, but her champion page does say that she’s a marksman and includes a brief description of her powers:
A headstrong, spirited young woman from Zaun’s working-class, Zeri channels her electric magic to charge herself and her custom-crafted gun. Her volatile power mirrors her emotions, its sparks reflecting her lightning-fast approach to life. Deeply compassionate toward others, Zeri carries the love of her family and her home into every fight. Though her eagerness to help can sometimes backfire, Zeri believes one truth to be certain: stand up for your community, and it will stand up with you.
This description makes it sound like Zeri will be a zippy, electricity themed ADC with quick attacks and movement. Of course, we’ll have to wait until Riot shows off her whole kit to know exactly what she can do.
Zeri’s champion page also reveals that she is from Zaun and her only related champion at the moment is Ekko. While there’s no way to know for sure, this certainly seems to hint that she could be an important part of Arcane season 2, or at least a part of Ekko’s life in Zaun with the family of misfits he protects and leads.
There’s no word yet on when Zeri will be released, but her kit should be revealed sometime in the next few days and arrive in the full game a couple of weeks later.