What's the difference between Off-Tanks and Main-Tanks ?

What's the difference between Off-Tanks and Main-Tanks ?

Because in many videos people say Main-Tanks and Off-Tanks but I never really understood the difference ???

of tanks are: roadhog dva and hammond and main tanks are winston rein and orisa
main tanks are meant to direct the team and off tanks to make space and peel for the healers

main tanks are the main ones ion the tank pack what do u think

A main tank is a tank w/ a shield
Offtank -> the others