What does it mean for a chemical pathway to be conserved?
A conserved pathway is a pathway that exists in a variety of species, by virtue of that pathway being conserved throughout the evolution of those species. A pathway must by neccessity have appeared for the first time in one particular species. If that species gives rise to new species, but the pathway in question is identical or very similar in all the new species, it is sensible to say that pathway has been evolutionary conserved. This may imply that the pathway in its conserved form is important for the fitness of those species.

For some examples, here is a page on evolutionary conserved pathways in Drosophila: http://www.sdbonline.org/sites/fly/aimain/aadevinx.htm