Jelly roll pans are about the size of a large rimmed baking tray, except the edges are higher — about 1 inch (2 1/2 cm) tall. They are used to make a big, shallow sponge cake that is then rolled up to make a jelly roll (aka Swiss roll.)
They can also be used for “tray bakes” to make bars of cookies, or layers for a large layer cake.
A normal size is 9 inches by 15 inches (23 x 38 cm.) A 12 by 18 inch (30 by 45 cm) size is referred to by bakers as a “half-slab.” (See: Baking tray sizes.)
To use for a jelly roll, follow your recipe’s instructions on how to prepare the pan (e.g. grease, grease and flour, line with paper, etc.) Pour the batter in, and bake, then let cool a bit.
Then, roll up the layer of sponge in a thin, flat slab. You need to roll it while the cake is still relatively warm, so it is pliable.