What is meant by the term 'sous chef''?

Executive chef is in charge of everything and everyone else... he's the head guy in the kitchen (cooking and making sure everything goes as it should). Sous mean "under" in English, so a sous-chef is under the executive chef (figuratively). So basically the sous-chef is second in command. This position is most likely doing whatever the executive chef doesn't want to or have time to do in regards to personnel and other kitchen duties and in the absence of the executive chef, the sous-chef would be in charge of the kitchen.

"sous" in french means under chef.

So "Sous Chef" is essentially the Assistant Chef. He will either assume the responsibility of the Executive Chef when the Chef is absent or assist the Executive Chef in his duties. The Executive chefs duties are quite extensive. Fundamentally in ost restaurants the Executive Chef will be completely in charge over everything in the kitchen and in some the entire establishment. This means purchasing food and paper product, Scheduling and labor costs, Menu creation, Cleaning Schedules/Assignments, training of cooks (and sometimes servers) and many more executive duties similar in nature.