Okay, as spoiler-free as possible:

The Red Angel is a mysterious entity or phenomenon witnessed by Starfleet in various locations around the galaxy. It was given its name by a person who had also seen it many years in the past.

Season 2 of the series is largely about the search for the nature of the Red Angel.

As the comments have pointed out, this link has more information, but even glancing at the page could give away important plot revelations: http://memory-alpha.fandom.com/wiki/Red_Angel

(Upvoted because it's a legitimate question, especially if the OP doesn't have time or access to actually watch the show.)

Okay, as spoiler-free as possible:

The Red Angel is a mysterious entity or phenomenon witnessed by Starfleet in various locations around the galaxy. It was given its name by a person who had also seen it many years in the past.

Season 2 of the series is largely about the search for the nature of the Red Angel.

As the comments have pointed out, this link has more information, but even glancing at the page could give away important plot revelations.