Basically, the current block size limit of 1MB isn't sufficient for the current volume of transactions. Some transactions aren't making it into blocks in real-time and have to sit waiting in a queue to be processed. This leads to delays in processing commercial payments, which in turn threatens Bitcoin's viability as a mainstream currency.
Some people argue the issue could be easily fixed by simply raising the block size limit, but others disagree and believe that would become too costly in bandwidth and processing capacity for miners, so AFAIK there's still no consensus on a resolution.
A couple other good explanations: http://rusty.ozlabs.org/?p=535 https://medium.com/@octskyward/the-resolution-of-the-bitcoin...