A stream key is a unique alphanumeric string some platforms use to identify your incoming stream. Examples include Facebook, YouTube and Twitch. An alphanumeric string is often used for security, because its hard to guess or randomly find.
Some platforms let you use a reusable stream key, meaning the same key can be used again and again. And some platforms either require unique keys for every stream, or require it depending on your event settings. Below is some guidance by platform on which ones allow you to use which type of key.
Understanding this is helpful because reusable stream keys are often more versatile - if your stream stops, for instance if you run out of camera battery and it takes you some time to replace it - you can just start streaming again to a reusable stream key. But a single use stream key may not be reusable and thus you might have to make a whole new event. The one downside of reusable stream keys is they offer somewhat lower security, since should your stream key be accidentally revealed, you would have to go change it proactively.
Facebook requires unique stream keys per post created using the Live API. They do allow reusable stream keys but only when you make the post in the Facebook user interface, and not in the Solo portal interface. If you want to use reusable stream on Facebook, use the Facebook user interface to make the post, and then use the Generic RTMP destination type in Solo, copying and pasting the ingress url and stream key that Facebook provides.
YouTube is a bit of a complex answer: Stream Now is a reusable stream key, and events can use a reusable stream key as well, what YouTube sometimes calls a Reusable Stream. Or you can opt to use a single-use stream key with Events. In both the YouTube interface and the Solo portal you can choose between the two, and select from a list of existing stream keys. But with this extra power to choose what you want comes a little complexity where it can sometimes be confusing which one you are selecting - be sure not to select "single-use stream" or "single-use stream key" if you want a reusable stream key.
Twitch only supports reusable stream keys!
A Note On Security
If you use a reusable stream key and you worry that your stream key was exposed or want to change your key (similar to how you might want to change your password), all the above platforms support that. In Twitch you can request a new unique key, which then becomes your reusable one. In YouTube and Facebook, you can just make a new event or post and choose at that time to get a new key, even if that key is itself reusable.