In a restaurant kitchen, to be "in the weeds", "weeded", or "in the shits" means that you are falling behind, you are disorganized, and your station is a mess. Being in the weeds is also a mental state. A cook in the weeds is likely unable to process basic information because they are under duress.
A single station in the weeds can quickly escalate to the entire restaurant falling behind. For example, if appetizers arent going out on time, guests are not going to be vacating their table on time in accordance with the restaurant's planned turn time. Entrees will be fired at an uncomfortable rate for the hot line resulting in reduced quality of what's being sent to the dining room and risking other stations falling into the weeds.
How to avoid falling in the weeds
Experienced line cooks and chefs will tell you that preparation and organization are they key to avoid falling behind. You should be properly prepared at the start of service so that you have adequate mise en place on station so that you never have to leave or do extra work on every pickup. You should work in a consistent manner asssembling every dish in the same order. And above all you should work cleanly taking a couple of seconds after every pickup to wipe down and make sure everything is back where it was before you started that pickup. if you find yourself starting to feel overwhelme, pause for a moment and take a deep breath, do a quick wipe down and then proceed one task at a time. In time you will be able to handle the pressures of your station during service better and you will develop a greater capacity to not fall behind.