Dm7 is built from D, F, A and C. D is the root, F the minor third, A the fifth and C the minor seventh.
String by string, low to high, it plays D, A, C and F. It runs from D on string 4 up to F on string 1.
In Dm7, only one string rings open, string 4, which gives D. Strings 6 and 5 stay silent. Your hand covers two frets, from 1 to 2. There is one more position, starting at fret 5.
Change only string 2 in Dm7 and you have Dm. Inside Dm7 sits a complete F chord, made of F, A and C. F shares three positions with this shape.
You will find Dm7 in five different keys. In F major it is the vi, in C major the ii and in Bb major the iii. The rest appear in the key list. Further down there is a song that uses it.