A fold change describes the ratio of two values, e.g. (gene expression in Condition 1)/(gene expression in Condition 2). If the expression was twice as high in Condition 1 than in Condition 2, there is a fold change of 2. Alternatively, if the reverse was true, there would be 1/2 fold decrease.
Fold change values are commonly log2-transformed. This is convenient for visualization purposes, as a log2 fold change of 0 means no change, and reductions have a negative value and increases a positive value.