That's when the block rewards officially run out. No more new Bitcoin. From that point on, the entire security of the network—the miners who protect it from attack—will rely only on transaction fees.
Miners only confirms a transaction that comply with the protocol of the bitcoin network, when you're talking of the security of the network, then first consider the nodes and then the protocols concessions, this is the basics upon which every transaction is built upon, miners till now are earning from transaction fees and mining, but by then, they will only have to depend on transactions being confirmed.