Plain English Meaning
When you are flying by looking outside (VFR), you have to stay a certain distance away from clouds. How far depends on which type of airspace you are in and how high you are flying. The point is to keep enough space between you and a cloud so that if another aircraft pops out of it, you both have time to see each other and avoid a collision.
Full Definition
The minimum horizontal and vertical separation a pilot operating under Visual Flight Rules must maintain between the aircraft and any cloud. The required distances are set by 14 CFR 91.155 and vary with airspace class and altitude, ensuring VFR traffic stays far enough from clouds to other aircraft that may emerge from them.