Translation:Who is standing next to that big yellow car?
All postpositions do this - if there's a demonstrative az or ez in front of the noun, then it has to receive the postposition as well. And it will lose the z if the postposition begins with a consonant.
a piros alma (the red apple)
a piros alma mellett (beside the red apple)
az a piros alma (that red apple)
a mellett a piros alma mellett (beside that red apple)
ez előtt a piros alma előtt (in front of this red apple)
e mögött a piros alma mögött (behind this red apple) and so on.