Jelly is a kind of wall/object. It's a purple gelatinous substance that just can be destroyed if the Mighty Eagle knocks it, it's hit by a laser or something explodes close to it (like a Black Bird).
Wall and Ball Jelly
Once a normal bird hits it's wall/ball version the bird will bounce and be knocked away from the screen, making it a very dangerous jelly type. Most pigs like doing big walls and hideouts to stop the birds, so the player should explode the jelly with Black Bird or try finding another way of getting to the pigs. If Ball Jelly is hit it'll roll away but the player will still bounce. The Ball Jelly may sometimes trap the a bird inside of it and crush it if it doesn't go fast enough.