MTU stands for Maximum transmission unit, which is the maximum packet length that can be sent on an egress interface toward a destination. MTU is an attribute of the egress interface and is typcially considered over a full path from the source to destination.
MTU size differes from one media to another and is often a source of issues when configured incorrectly in the network. The following posts will give deep understanding for What is MTU and how you can manipulate it and set it to the correct value.
The post starts from the very basics and defines exactly MTU and MRU (Maximum receive unit) and the different ways to manipulate them in a production network. In this post we also clarify PMTUD (Path MTU discovery ) and TCP MSS (Maximum segment unit) clapping and how they are used to solve or avoid problems caused by MTU.
MTU and ping size confusion is another interesting post on the different implementations of network operating systems and how they can cause confusion to network engineers based on how they interpret their ping commands.
MTU is a very simple concept, but needs to be understood throughly, specially in service provider environments and multivendor equipment. the misconfiguration or MTU or path MTU discovery (PMTUD) is a big source of problems that are some times very hard to spot and fix because of the different behavior they exhibit on different paths from sources to destinations.
MTU settings mismatch on neighbors interfaces can have an impact on IS-IS neighbor adjancency relationships. Mounir has explored this topic in this post, just in case.
Here is also a great tool, that will help you visualize this process of MTU size calculation requirements based on what type of headers you expect on the network path. Make sure to bookmark it for future use.