Quarantine is for people who are not currently sick but who have been or may have been exposed to a communicable disease, like COVID-19. If you have been in contact with someone who has been placed on quarantine: they are not contagious---yet. They are being monitored for symptoms and are not identified as a COVID-19 positive case. Your contact with a quarantined individual does not put you at risk. Quarantined individuals have been in direct contact with a positive COVID-19 case but may not have enough virus in their system to be contagious, or even develop the illness at all. Should that quarantined person develop symptoms or test positive, their contact with you within the investigation timeframe could be considered in their case investigation.
Isolation happens when a person is infected with a communicable disease, like COVID-19, and is separated from people who are healthy.