In live streaming settings, H.264 is an important feature to set as it is a video encoding format that makes it possible to provide good video quality with much lower bitrate than previous standards without increasing complexity.
One of the settings that can be changed for H.264 is the profile. The most common profiles are Baseline, Main, and High.
What sets these apart is that they deliver lower bandwidth, the higher you set the value. In theory, you should get the same quality with lower bandwidth if you go from Baseline to Main or from Main to High.
With higher video compression, more performance is required, which means that this is one aspect to take into account for live broadcasts to not go beyond the performance you have available. In such a scenario, the quality coming out of your computer or encoder may be poor even before Internet connections become involved.
In practice, most reasonably modern devices are capable of High today, but if the audience is unknown, Main is a better choice as it works with slightly older equipment. Baseline is still common and hassle-free.
As a rule of thumb:
If you are broadcasting to unknown audiences with likely varying equipment and connectivity. Select Baseline. The same can be good if you have the world as a market.
Main is a good choice for quality streaming when there is reason to believe that the audience is well placed to receive streams.
High is best suited when you know that there are good conditions, currents in high quality against modern equipment.