Orbital debris covers stuff large enough to be monitored by U.S. Combined Space Operations Center’s (CSpOC) Space Surveillance Network (SSN). It also covers stuff too small (from about 5mm to about 30cm) to be monitored by CSpOC but large enough to pose risk to human and robotic space missions. This smaller debris would be monitored by ODPO.

https://orbitaldebris.jsc.nasa.gov/measurements/radar.html

> NASA's main source of data for debris in the size range of approximately 5 mm to 30 cm is the Haystack Ultrawideband Satellite Imaging Radar (HUSIR). The HUSIR radar, operated by the Massachusetts Institute of Technology’s Lincoln Laboratory, has been collecting orbital debris data for the ODPO since 1990 under an agreement with the U.S. Department of Defense. HUSIR statistically samples the debris population by "staring" at selected pointing angles and detecting debris that fly through its field-of-view.

> The data are used to characterize the debris population by size, altitude, and inclination. From these measurements, scientists have concluded that there are approximately 500,000 debris fragments in orbit with sizes down to one centimeter.

https://orbitaldebris.jsc.nasa.gov/faq/#

> 3. How much orbital debris is currently in Earth orbit?

> More than 23,000 orbital debris larger than 10 cm are known to exist. The estimated population of particles between 1 and 10 cm in diameter is approximately 500,000. The number of particles larger than 1 mm exceeds 100 million.

Orbital debris is already causing damage. Here's some examples from Hubble: https://orbitaldebris.jsc.nasa.gov/measurements/in-situ.html

The photo gallery shows other examples of impact damage as well as examples of debris: https://orbitaldebris.jsc.nasa.gov/photo-gallery/