Adenine always pairs with thymine when forming chains of DNA. In RNA, a similar molecule to DNA that is also used to encode genetic information, adenine always pairs with uracil.
Adenine is a nucleobase. The DNA molecule is shaped like a ladder, and pairs of nucleobases, one from each side of the ladder, form the rungs. The bases encode genetic information.
The reason why adenine pairs with uracil in RNA instead of thymine is possibly because uracil is easier to make than thymine. RNA is typically used for the transfer of genetic data, while DNA is used for permanent storage. Making multiple copies of RNA, utilizing a more energy efficient nucleobase, eases the transfer process. Another reason why thymine may be adenine's pair in DNA, rather than uracil, is that it makes repair of damaged DNA easier.