Short Call / Short Put

Short Call (You think the underlying will fall or stagnate in the future)

You are obliged to sell an underlying at some point in the future if the option holder exercises his option. You think that the underlying price will fall or stagnate in the future. If the underlying falls the buyer won’t exercise his option and you stay with the premium. If the underlying rises you might get into trouble.

Short Put ( you think the underlying will increase or stagnate in the future)

You are obliged to buy an underlying at some point in the future if the option holder exercises his option. You think that the underlying price will increase or stagnate in the future. If that happens the option holder won’t sell his underlying for the lower exercise price of his option and you stay with the premium.