Uncore frequency is the frequency of the non-core parts of the CPU, ie cache, memory controller, etc. It's also known as ringbus frequency.
Opinions are somewhat divided on how much effect it has on overall performance (if any). What is certain is that it's completely pointless to set uncore frequency above core frequency.
I'd recommend getting your core up as far as you can without touching uncore. When you're done, up uncore to as close as you can get it to core frequency. Don't sacrifice core frequency for uncore frequency.