Spp is the abbreviation for the plural form of the word species. (which is, presumably, also "species").

The most common use of this abbreviation is when describing the Latin name of a plant or animal that compromises many species. For example,

The Rose (Rosa Spp) is a common flower in north temperate latitudes.

Since there are about 200 different species of rose in the genus rosa, the abbreviation spp means all of them.

And since it is part of the Latin name, the letters should always be written in italics.