Stereochemistry relates to the three-dimensional arrangement of atoms and molecules and the effect of this spatial arrangement on chemical reactions. Stereoselectivity is the preference for a reaction to occur on one molecular face over another, resulting in the preferential formation of one stereoisomer. If there's more than one stereoisomer possible from a reaction, and the configuration of at least one chemical bond is not known, that bond can be represented with a wavy bond.
Figure 1. Stereoselectivity in bromide ion addition to a carbocation. Note the solid and hashed wedged bonds.
A reaction is said to be stereoselective if the reaction mechanism selects the formation of one stereoisomer over the other. In contrast, a stereospecific reaction is a reaction which only allows the formation of one stereoisomer (e.g. the stereochemistry is already set by the starting material).