Camber is the slight upward curve a prestressed precast member takes on when the strands are released. The prestressing force pulls the piece into a gentle arch before it ever leaves the plant.
That is by design. Once the member is set and carries its load, the camber flattens out and the piece sits level. Estimating camber correctly matters for fit and for how a member looks in place, since too much or too little shows up on the job site.