In the M/M/1 queuing model, the letters "M" and "1" denote specific characteristics of the queuing system. The first "M" stands for "Markovian" or "memoryless," indicating that the arrival process follows a Poisson distribution, where events occur independently and randomly over time. This implies that the time between arrivals is exponentially distributed. The second "M" also represents a Markovian process, indicating that the service times are also exponentially distributed. The "1" signifies that there is a single server in the system. Therefore, the M/M/1 model describes a queuing system with Poisson arrivals, exponential service times, and one server. This model is commonly used to analyze and predict the performance of systems such as customer service lines or computer networks.