Linebreeding is when the same ancestor appears in both the sire's side of the pedigree and the dam's side of the pedigree. The numbers refer to how many generations back, sire's listed first. So a 3-4 linebreeding means that Dog X is in the 3rd generation back on the sire's side and 4th generation back on the dam's side.
Linebreeding concentrates the genetics of that specific dog, both his good points and bad points. Some linebreedings are coincidental rather than intentional, some are intentional. Sometimes there is a strong presence of the linebred dog in the pups, sometimes there is very little influence seen especially with linebreedings farther back. Whether it is good or bad depends on the dog being linebred on, his strengths and weaknesses, how the rest of the pedigree balances out those strengths and compensates for those weaknesses, and ultimately the knowledge of the breeder doing the linebreeding.