Examples of use
The maj7 or Major 7th chord (sometimes written as △7 to avoid confusion with minor chords - eg. Db△7) is usually found on either the I or the IV (1st of 4th) degrees of a major scale.
So in Db major, this would be Dbmaj7 or Gbmaj7.

In slightly more jazzy circumstances, you might play a maj7 chord on a more chromatic note of the score, as in this example, in Bb major:

Major 7th chords are one of the Diatonic 7th chords we talk about in the theory lesson here.