A Lunar Tetrad is series of four total lunar eclipses in a row, which have no partial lunar eclipses between them.
There are 6 lunar months (full months) between them. The 21st century will have a total of 9 Tetrads.
Each total lunar eclipse causes the Moon to appear an eerie blood red color, also known as a Blood Moon. You can read more about those here: What is a Blood Moon?
This short video from Nasa explains in more detail what a Total Lunar Eclipse and Lunar Tetrad is, beginning with the first total eclipse in April 2014, then October 2014, April 2015 and finally September 2015:
I hope this has helped explain in more detail what a Lunar Tetrad is, and if you’ve any questions, please leave them below.