In string/M-theory, the multiverse corresponds to the number of different solutions to the dynamic equations. Each member of this multiverse is characterised by the specific way the space of the compactified extra dimensions looks like. The shape of the extra dimensions determines the details of the laws of nature, so each member of such a multiverse is governed by its own law of nature, number of large observable dimensions, etc ... However, the fundamental laws of quantum mechanics and general relativity must hold for all of them.
The appearance of this large space of solutions in string theory is a natural and legitimate thing (nothing unphysical or unscientific about it). Investigating the properties of this so-called string theory landscape is a valid part of actually ongoing research in theoretical physics. The goal of these efforts is to narrow this huges space of allowed solutions of the dynamical equations further down using mathematical and physical methods to find these smaller parts of the landscape which could actually describe the laws of nature and observation which characterize our universe. It is a very complicated task, some difficulties that come along with this undertaking are addressed in this question. 675
However it is not clear if each point in this space of solutions should be interpreted as a "universe" that really exists. My guess is that one should take this interpretation rather not too seriously.
In addition, as can be shown by some holographic arguments, it is not really meaningful to talk about things that could exist outside our own cosmic horizon (or universe). The inflating univers can be (mathematically) described by a notion that presumes that galaxies and everything else is falling (accelerated) towards the cosmic horizon. In this picture, the cosmic horizon is analogous to a black hole horizon, such that the inside of the black hole lies outside our visible cosmic horizon. The holographic principle then says that what lies outside the cosmic horizon (inside the black hole) can not be described independently from what is on the cosmic horizon (the event horizon); the two descriptions are equivalent. So it is physically meaningless to talk about what lies outside our own universe.
In summary, one can say that the string theory landscape taken just as the space of solutions to the dynamical equations itself is nothing unscientific and further investiations of its properties are part of valid current research in theoretical physics. But to imagine that there really exists another universe corresponding to each point in the landscape is in my opinion (and many string theorists say the same) probably not appropriate for the reasons I explained above. Accepting the string theory landscape just as a space of solutions but rejecting the physical existance of any corresponding parallel universes does not contradict each other.
Aside: <PERSON>'s many-worlds multiverse is just an interpretation of quantum mechanics and has nothing to do with what is explained above. 43