This refers to the probability that any particular calculation will find and solve a valid cryptographic block that generates bitcoins. The Bitcoin network is designed to generate one of these blocks approximately every 10 minutes. As more miners and computing resources are applied to the network, the difficulty in finding a valuable block is automatically increased. This prevents the increased resources from finding a block in less than the desired 10 minute period to maintain the steady drip of bitcoins entering circulation.
Malone, J.A (2015). Glossary of Bitcoin Terms and Definitions. United States: Lulu Press, Inc