Overview
Maximum Transmission Unit (MTU) is usually measured in bytes. For standard Ethernet, the MTU is 1500 bytes.
Importance
If a packet is larger than the MTU of a link it needs to cross, it must be fragmented into smaller pieces, which adds overhead and can cause performance issues. If the 'Don't Fragment' (DF) bit is set, the packet will be dropped, and an ICMP error will be sent back.