Hello Aditya
Traditionally, routers, switches, and computers require a different type of Ethernet cable to connect to each other. There are crossover cables and straight through cables, as described in the lesson.
What auto MDIX does is it automatically detects the kind of cable required for a particular connection, and if the cable used is the wrong type, then it automatically switches the appropriate pins internally on the switch or router, such that the correct connectivity is achieved. So for example, if you use a crossover cable to connect a PC to a switch (which is the incorrect cable type) the auto MDIX feature will detect this and internally switch the Tx and Rx pins so that the connectivity is achieved.
I hope this has been helpful!
Laz