What is left basal lung discoid fibrosis?

Benign. This is "radiology talk" describing linear areas of lung with some scarring. Very common and nothing to worry about. In truth, someone your age likely has discoid atelectasis rather than fibrosis. Atelectasis just means the lung is not inflated as much as usual and this can be seen when a patient hasn't taken a deep breath for the xray or after epidsodes of pneumonia.
Sounds like. You are reading a chest x ray report. Ask the doctor who ordered the test, but it is probably scar from old infection, not active pneumonia.