M/M/1 means memoryless exponential inter arrival times/memoryless exponential service/one server.
Suppose that the inter-arrival times are exponential of parameter A, and the service times are exponential of parameters D.
Then the number of customers in the queue X evolves as Markov Chain.
Suppose at time 0 there are N customers in the queue(X) where X>=0.
Denote by T the time taken to serve the first customer and by L the time of the next arrival.
If T> A X=N+1
If T< L X=N-1
The case where N=0 there is no serving going on.

In our case ,we call by :
Time of Simulation:working time of the system/store (number of hours,minutes etc...)
No Of Arrivals :Number of customers who arrive per unit of time(Arrival Rate)
No Of DeparturesNumber of customers who get served and leave per unit of time
No Of the Queue:number of customers in the queue @ time t.