There's about a 10% chance that two non-functional spacecraft will have an extremely high-speed collision.
They're at about 1000 km altitude, near the iridium orbit height. Objects of this size, approaching almost head-on, could create a significant cloud of dangerous debris.
The objects are a 1999 Chinese 3rd stage rocket and a 1989 Soviet military satellite. Neither are under control so avoidance is not an option.
The projected path has them missing by about 40 feet as they cross at an approach speed of about 32,000 mph. But the uncertainty is about 60 feet, hence the risk at 10-20%.
The Soviet satellite has a 56 foot boom attached, that's one of the reasons for the relatively high risk.
The crossing will happen at 8:56 PM ET tonight, over Antarctica just south of South America. A worst case scenario would have them breaking up into hundreds of smaller pieces still carrying similar speeds.