A left shift of a CBC refers to an increase in the number of immature WBCs. For example, during times of inflammation or infection the turnover rate for neutrophils is increased which leads to (premature) release of immature neutro's (bands) from the bone marrow.
On the CBC differential you will your blood cell counts with neutro's further divided into bands and segs. Seg's are the mature form with their segmented nuclei. Bands are the immature form whose nuclei are unsegmented and look "band-like". When the band value is increased above reference range this is a left-shift.
As far as right-shift goes, I'm not certain. Intuitively, it may mean an increase in number of mature cells. But I can't vouch for that. Another poster will hopefully help you out on that one.
-P.