Voyager is a Lemmy/PieFed frontend. Kind of like how if you have a Gmail account, you can check your email using the official Gmail app or with any alternative email app like Thunderbird, K-Mail, Fairmail, the iOS Mail app, etc. if you want. Those apps are all just front ends for accessing your email account. Similarly, you can access Lemmy/PieFed via frontend apps like Voyager, Thunder, Blorp, etc.

Note that while many apps support both Lemmy/PieFed, some apps only support one or the other. For example, Jerboa is just a Lemmy app and does not support PieFed in any way.