Bias Tees are RF components that are used whenever you need to couple a DC, power or low-speed control signal onto an RF signal path. This video discusses the basics of Bias Tees, how they work, examples of practical construction and a few lab application examples showing them "in action". Links for the Bias Tees used in this video, as well as a copy of the video notes are here:
https://www.minicircuits.com/pdfs/ZFBT-4R2GW+.pdf
https://www.minicircuits.com/pdfs/ZX85-12G+.pdf
http://www.dorkage.com/youtube/Basics_of_Bias_Tees.pdf