I think the term 'chronic' here refers to continuing lyme, extending beyond and despite treatment.

IDSA (Infectious Disease Society of America) and the CDC don't believe in chronic lyme. They call it Post-Treatment Lyme Disease Syndrome. Per CDC:

Approximately 10 to 20% of patients treated for Lyme disease with a recommended 2–4 week course of antibiotics will have lingering symptoms of fatigue, pain, or joint and muscle aches. In some cases, these can last for more than 6 months. Although often called "chronic Lyme disease," this condition is properly known as "Post-treatment Lyme Disease Syndrome" (PTLDS).


As for the symptoms and complications of untreated (never before treated) lyme, the CDC calls it 'Late Dissemminated" stage-per the CDC:

Late disseminated stage (months-to-years post-tick bite)
Approximately 60% of patients with untreated infection may begin to have intermittent bouts of arthritis, with severe joint pain and swelling. Large joints are most often affected, particularly the knees3. Arthritis caused by Lyme disease manifests differently than other causes of arthritis and must be distinguished from arthralgias (pain, but not swelling, in joints).

Up to 5% of untreated patients may develop chronic neurological complaints months to years after infection4. These include shooting pains, numbness or tingling in the hands or feet, and problems with short-term memory.

I think the reason why it's so confusing is again the controversy with the IDSA guidelines, which the CDC follows. So chronic lyme technically doesn't exist, yet sufferers know all too well that it does.