Q: What is worse than raining cats and dogs?
A: Hailing taxis!
Explanation: Precipitation is water in any form that falls from the sky. This includes rain, snow, sleet and hail. Hail is frozen rain, ice, that falls; it can be very small or quite large.
Hailing a taxi means to get the attention of a taxi driver so that you can get a ride somewhere. Hailing taxis also sounds like taxis are falling from the sky like frozen rain, like hail.
Raining cats and dogs is an idiom meaning that it is raining very hard, that the rain is pouring down.
This joke is funny because it takes the idiom raining cats and dogs and makes it precipitate harder by hailing taxis. This joke plays with the word hailing, meaning to call a taxi from the side of the street, or falling ice.
Here is Bob Dylan singing A Hard Rain is Gonna Fall: