It means you have a memory kit that is rated at 3733mhz, it may or may not work at 3733, it may or may not work at 4000, ultimately it depends on the IMC, if it runs at XMP rating then you can try to go up from there, if it doesn't you will have to adjust some voltages or settle for less than rated speed, you will not know until you test it
Check board specifications for memory speed ratings