What does it mean for the signals for transcription and translation to be "conserved"?
I concur with @Bez but wish to elaborate on the meaning of 'conserved'. It is generally used in the context of evolution. A conserved characteristic or gene or protein means that it has 'survived' a long time without being altered. As @Bez mentions, certain parts of the protein machinery in eukaryotes is very different from prokaryotes, the latter being considered to have arisen much earlier in evolution. For example, some post-transcriptional processes (e.g. splicing and many more differences) only occur in eukaryotes. But there are many similarities as well, such as the basic use of mRNA etc (see book section). The latter are examples of conserved mechanisms.  In the context of your question, it means that when you take eukaryotic DNA to express eukaryotic proteins it doesn't really matter which cell or host organism you take as it will be transcribed and folded correctly. For example, yeast is often used to express DNA from higher organisms as its protein synthesis is much the same across various eukaryotic species. However, expressing eukaryotic DNA in prokaryotes likely ends up in misfolded proteins without proper post-transcriptional modification and should be avoided. 

As to your follow-up question: Folding of proteins happens in the endoplasmatic reticulum (ER) in eukaryotes (wiki), a cell organelle absent in prokaryotes as they lack a nucleus and associated organelles. Again, expression of eukaryotic genes in a prokaryotic expression system will end up in mis-folded proteins.     

To get back to your overall question - the 'signals': DNA transcription, splicing, protein synthesis, post translational modification and folding are all regulated by signals, such as specific DNA codons to start and stop transcription, RNA codes for splicing and start translation and protein tags that are placed on specific amino acid combinations. These signals are well conserved across eukaryotic species.