They are regular yellow noodles in a glass jar.
Role in series
"SpongeBob You're Fired"
SpongeBob applies for a job at Wet Noodle and cooks a Noodle Patty. Noodleman fires SpongeBob due to his poor cooking. However, he later fights for him among the other three restaurant owners since his customers apparently liked the Noodle Patties.
"The Legend of Boo-Kini Bottom"
When SpongeBob is doing the "Jars of Horror," he describes the noodles feeling like worm guts. Sandy later explains that they are just noodles.
- Noodles on Wikipedia