Quarantine means restricting activities or separating people who are not ill themselves but may have been exposed to COVID-19. The goal is to prevent spread of the disease at the time when people just develop symptoms.
Isolation means separating people who are ill with symptoms of COVID-19 and may be infectious to prevent the spread of the disease.
Physical distancing means being physically apart. WHO recommends keeping at least 1-metre (3 feet) distance from others. This is a general measure that everyone should take even if they are well with no known exposure to COVID-19.