The DB3.1 strain of E.coli provides resistance against ccdB protein, due to mutation in gyrase gene. While general E.coli strains like DH5alpha are sensitive to ccdB protein. In gateway cloning; use of ccdB will help us in differentiating between desired clone and original vector. In absence of such selection both destination vector and LR product will survive in E.coli and it will be very difficult to select only the insert containing clones in destination vector.

@Kamal  Thanks for the explanation! I also find this web useful: http://blog.addgene.org/plasmids-101-ccdb-the-toxic-key-to-efficient-cloning