A maximum transmission unit is the max size a given protocol can transmit in a single transaction in a network.
To this day, the Ethernet MTU is 1500 bytes. This is a relic of the Xerox Alto computer.
if you add the Ethernet header – 36 bytes – then an MTU of 1500 plus that header is 1536 bytes, which is 12288 bits, which takes 2^12 microseconds to transmit at 3Mb/second, and because the Xerox Alto computer for which Ethernet was invented had an internal data path that ran at 3Mhz, then you could just write the bits into the Alto’s memory at the precise speed at which they arrived, saving the very-expensive-then cost of extra silicon for an interface or any buffering hardware.