The document describes a CALCULATE function that performs arithmetic calculations on a stack. CALCULATE pops operators and operands off the stack and performs the operations, pushing the results back onto the stack. It will pop two integers, perform the operation indicated by the popped operator (+, -, /, etc.), and push the result back onto the stack. The example code provided enqueues values and operators onto a queue, converts the queue to a stack, and calls CALCULATE to perform the calculation.