IO limits restrict the data throughput for the customer. They are in KB/s. When limit is reached, the processes are throttled (put to sleep). This makes sure that processes within LVE cannot go over the limit,. Yet don't stop working, nor getting killed -- they just work slower when the limit is reached.
IO limits are available with kernels el6.lve1.x and higher.
The IO limits will only affect DISK IO, and will have no effect on network. It also doesn't take into consideration any disk cache accesses. So, even if file is loaded from disk cache 1000 times -- it will not be counted towards IO limits.