If your existing drain flange is 3/4" NPT, you need this to reduce it down to our lights. Which is 1/2" NPT.
NPT stands for = National Pipe Thread
3/4" NPT physically measures 1"
1/2" NPT physically measures 3/4"
NPT threads are tapered and seal as you tighten them. Teflon tape or TefGel (found HERE) will keep the threads protected, sealed and help prevent corrosion.