BiDi optics use a single fiber to connect between devices while standard optics require two fibers. Note this means that with BiDi the transmit and recieve run on the same fiber so they have two frequencies noted.
I'll also note here that SM optics come in a wide range of distences and frequencies you do need to read the specs on each and match the pairs on the link. Distances can range from 10-100 km (120 for xfp). You will need to match both distance and frequency on the link.
Also using vastly larger distance rating than needed can cause the link not to work. There is a hot alarm on optics if they recieve too much power and the link won't come up. So you would then need to pad the link in that case. The distance number is a substitute for the transmit power and these are just estimates, your milage may vary based on quality of fiber, splices, connectors and patch panel usage.