Just to talk about residuals a bit more. In most industries, when a company produces something they need to pay labourers some money. Every time a a company makes a chocolate bar or a TV or a car, labourers are paid some money. After that item is sold, the company needs to make a new one and so labourers are constantly being paid.
Movies and shows are different. Labourers are paid to make it and then the company can sell it an infinite number of times without paying anyone anything. It’s a digital file. This means companies can profit off the labourers’ work for decades without paying them any more than the initial cost. Residuals are a way to make it so entertainers are paid similarly to any other labourer. Each time the movie company makes a copy, they have to pay the labourers a small amount of money. That way if the company profits for decades, so do the people who actually appear in it.
The actors are on strike because their performances are being played over and over and over and customers pay the company over and over and over and the actors only get paid once.