Nsight profiler, memory becoming bottleneck

I’m reading the Nsight Profiler Guide, and at some point ran into this statement:

Memory can become a limiting factor for the overall kernel performance when fully utilizing the involved hardware units (Mem Busy), exhausting the available communication bandwidth between those units (Max Bandwidth), or by reaching the maximum throughput of issuing memory instructions (Mem Pipes Busy).

Can somebody explain briefly what “Mem Busy” actually means, exactly? In what case does this thing come up?
I have a good understanding of what Max Bandwidth means, and a pretty decent idea of what Mem Pipes Busy means (although any thoughts would be greatly appreciated in that regard as well)