From Envirowiki

Diversity is a state in which some class has a lot of variation within it. For example, the pages on wikipedia are extremely diverse: they cover many, many topics; Conversely, a plantation forest is usually a monoculture, which is the epitome of non-diversity.

Diversity is important as a basis for stability and security. For example, a publishing company which specialises in mills and boon romance novels is subject to the demand for such material, and as such is inherently unstable, and insecure. Another publisher who doesn't specialise, but has a finger in every pie is inherently more stable, as the literature market as a whole is more stable and less prone to fluctuations than the mills-and-boon market. The same principle can apply to any field, and certainly applies to ecosystems. This is called biodiversity.