Codebook Preview provides detailed information about each variable. The Codebook tab provides the same information and also allows you to move from question to question within the codebook.
In a variable list (either in the Variable Search tab or the Review Selected Variables tab), you can move your mouse pointer to hover over the RNUM or question name of a variable. The codebook page will then show in a small pop-up window named the "Codebook Preview" (as shown below for an NLSY97 variable that describes highest grade completed).
See Codebook tab for description of codebook contents.