Answer:
1) He discovered penicillin.
2) He accidentally discovered it when he went on vacation and came back to a green mould called penicillin had contaminated the Petri dishes in his lab and found that it was skilling the bacteria he had been growing.
3) Penicillin is extremely important. Wounds that would be fatal back in the day could be quickly treated with penicillin. Penicillin also inspired others to test other natural products that are now used today.