DVDs are coded to play in specific regions of the world. Your DVD drive is also set to a particular region, generally the region where you live, although if you brought the computer or DVD drive from a different country to where you normally live, then that might explain the problem. Or, if the DVD's come from a different region, you will see the same problem.
If the DVDs are set to a region different to that of your DVD drive then they will not play. If you open the computer's Device Manager, find the CD/DVD ROM entry, and open its Properties window, you can check the Region tab and see if it matches that for where you are. You can change the region there, but beware, you are only allowed 5 changes.
If the DVDs are from a different region, then perhaps you need to obtain them for the region where you live.