Actually, it suggests that the poster has discovered either by reading or by trial and error, that the "5V" pin on an Arduino (with certain exceptions) is not in any way usable as a source of 5 V.
So you need a regulated 5 V power supply - a USB "phone charger" is often the easiest source - connected to supply the "5V" pin and ground as well as to all the other 5 V devices.
It is advised for a UNO to disconnect the "5V" pin (but of course, not the ground) whilst connected to a PC.
All grounds connect together - what other problems might you have?