Brief: Its a messaging technique for queued communication:Detailed: MSMQ. The queued transport in WCF uses MSMQ for its queued communication. ... The MSMQ queue managers implement a reliable message-transfer protocol so that messages are not lost in transmission. The protocol can be either native or SOAP-based, such as the SOAP Reliable Message Protocol (SRMP)Fore more go to MS Docs Link: https://docs.microsoft.com/en-us/dotnet/framework/wcf/feature-details/queuing-in-wcf

The MSMQ means Microsoft Message Queuing, is the feature allows us to create a program that can reliably send or receive messages over a wire without any loss of message. It makes use of a queue to store messages in the order in which they arrive. The queue is a First In First Out stack i.e. the message that arrived first will be the first that to be removed from the queue for processing by the server.You can visit the below link for more details: https://www.c-sharpcorner.com/UploadFile/17e8f6/msmq-in-wcf/

MSMQ is Microsoft Message Queuing if you want to handle message coming from high throughput system you don't directly expose your Services/API to such system. MSMQ is setup between the system generating message and the service. The system throws the message to MSMQ which is then consumed by the service on the frequency configured for the service. MSMQ can be configured to hold the message for defined time.