The Bitcoin network currently has a capacity limit on the amount of transactions that can get included in the blockchain within a certain time period. This limit is currently 1 megabyte worth of transactions per 1 block, with miners producing a block roughly once every 10 minutes.
The rise in popularity of Bitcoin, as a payment method and currency, is causing an increase in the amount of transactions, and creating competition between transactions to get included into the next block in the blockchain.
Since every transaction needs to pay a fee to miners for getting included, miners have an incentive to prioritize transactions which pay them a higher fee. This means that users that send an incorrect low fee, will experience significantly longer confirmation times, and in many cases, may not get included in the blockchain at all.