Slash chord, „ major seven over [maj7]”. On the left side of the slash you can see a chord, on the right side the lowest tone (bass).
The maj7 chord consists of the tones  (1),  (3),  (5) and [maj7] (maj7). Thus, the bass [maj7] is part of the chord. Since the bass on the right of the slash does not necessarily be part of the chord symbol on the left, the maj7/[maj7] can also be notated as /[maj7] chord. Depending on the context (chord progression) you may prefer either notation. You'll find /[maj7] and maj7/[maj7] with about the same frequency.
[maj7] [maj7]m7 (=/[maj7]) [b2]/[maj7] /[maj7] [maj7]
You can find this chord progression for example in the jazz standard "On Green Dolphin Street". Several major chords are played over a static root („pedal bass”) here. In this case the notation /[maj7] is favorable over maj7/[maj7].
This chord is often used in ballads, where a descending bassline, but little harmonical movement is desired. For example, in the chord progression
 maj7/[maj7] m7 m7/.
In this progression only the bass is moving. The m7 can also be seen as / chord. And m7/ in most cases is played as / chord here, because the root is continually descending and therefore is often not played anymore in the m7/ chord. Consequently you can alternatively write this progression
 /[maj7] / /
 maj7/[maj7] 7/ …
In this case the notation maj7/[maj7] helps to quickly recognize the way the chord types are changing. When playing a guitar solo that for sure can accelerate the decision which scales, arpeggios or guide tones to play. For maj7/[maj7] and 7/ you can even use chord shapes where the root  is not even played anymore and therefore all tones  of the first chord shape descend by a half-step ( C → B → Bb ).