when I load fritz 17 in chessbase, it often shows depth as for e.g. 18/25. what does this mean?
This means, that your searching UCI engine has currently reached the nominal depth of 18 plies but reached somewhere a maximum depth of 25 plies. Compared to the overall search depth the maximum depth should be of relative little importance, since it talks about a singular event among billions of nodes. Another problem is, that the UCI protocol (http://wbec-ridderkerk.nl/html/UCIProtocol.html) is deliberately extremly weak regarding the meaning of engine output. That makes sense, since search engine development still makes progress at a fast pace. If you want to know exactly what maximum it counts, you have to dig it up in the respective engine source code.
Hence if you want to report engine results, you can ignore the maximum depth reached, but should never ever miss to tell the nominal search depth.