It is log2-transformed ‘RMA-normalized data’ based on microarray. Since it is log2, a difference of 1 unit between genes and/or cell lines is similar to a 2-fold difference. For instance, Gene X has an expression value of 3 in Cell line A and an expression value of 4 in Cell line B. The difference is 1 unit (= factor 2), so the expression of Gene X is 2x higher in Cell line B than in Cell line A. A difference of 2 units means a 22 = 4-fold difference in expression; a difference of 3 means a 23 = 8-fold difference in expression.

As you see, the values are relative values, meaning that the values can be used to compare genes and cell lines. The values cannot be used in an absolute manner, for instance ‘1 ug of my sample contains 10,000 mRNA transcripts of Gene X’.