Processors work alot faster then RAM. Timings are the delays your processor must wait while communicating with your RAM. These delays are the time it takes for RAM to process data. The smaller the delay, the faster data will process. Which is all about megabytes per-second.
When you OC, data will be processing very quickly from your cpu, thus you have to loosen timings. Which means you need bigger delays, since more data is streaming at a faster rate from your processor to your RAM. Get what im saying..? Yes it is in your BIO's.