I have downloaded iso image of Ubuntu 14.04 from the official website. After that I burned it on a DVD, when I boot my computer with the DVD inserted in the drive I see the following messages on a black screen:
I do not know how to deal with them.
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You should make sure you're comparing against the correct hash, though. The hashes here are missing for some of the newer iso files. In particular, it doesn't currently have the md5 hashes for the 14.04.1 images, which is what you likely downloaded if you downloaded 14.04 recently.
http://releases.ubuntu.com/14.04.1/MD5SUMS includes the MD5 hashes of the 14.04.1 images.
You should download the iso again, md5 test the new one, and assuming that one is good, burn a new DVD from it. If the .iso file has a different md5sum from what it's supposed to have, that means it got corrupted, most likely during download. (Or it's possible the download didn't finish all the way.)
Corruption in file downloads is the exception rather than the rule, so most likely this second download will work. You should md5 test it too, though.
But if the md5sum of the new iso is wrong too, then provided you're comparing it to the correct official md5sum, that suggests a problem with the quality of the Internet connection used to download it. You could then use bittorrent or zsync to download it more reliably.