Entropy is a measure of the disorder or randomness in a system. In cryptographic systems, random numbers are incorporated during the creation of keys and certificates, in order to maximize uniqueness and minimize the possibility of breaking the encryption.
The random numbers used by crypto and communications applications can be derived from various sources. One commonly used source is a "pool of entropy" that is gathered and maintained by the system. Such a pool (for example, /dev/random or /dev/egd-pool) includes contributions gathered from the timing of device events, which are further hashed or subjected to other mixing and manipulations.
When a sufficiently large pool of such arbitrary data is gathered, the pool is considered to contain sufficient entropy, and numbers extracted from the pool, by recognized means, are considered sufficiently random to be used in strong cryptographic applications.
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