Theoretically, the loss in a short option position can be unlimited, but the loss in a long option position is limited to the premium paid.

Example: Suppose if we buy a call option on Dell with a strike price of $80 by paying a premium of $2 per share (Total $200 for one call contract). The maximum amount of money we will lose is $200 if the stock expires less than $80.

On the other hand, the short option position strats losing money if the stock price goes above $80 at expiry. The stock can go to any price above $80, theoretically.

If for example, the stock expires at $200, the the loss to the short option position = 200 - 80 + 2 = $122

If the stock expires at $500, the loss to the short option position = 500 - 80 + 2 = $422

And, now you get the point. The potential loss to the short option position is unlimited. To make sure that the option seller has money to make good for the losses margin is required.

If the stock price goes to 0, the maximum the long option trader loses is the premium paid, $2. This is paid upfront, so there is no need for margin.