Bethesda are best-known for their two ongoing, open-world RPG franchises: the Elder Scrolls (set in a fantasy world) and Fallout (set in a post-apocalyptic setting). Starfield is significant as the first new IP launched by Bethesda since releasing the first Elder Scrolls game, Arena, way back in 1994; they acquired the Fallout IP from Interplay, who had previously produced two main games and two spin-offs, in 2004.
Bethesda have been working on Starfield since the release of Fallout 4 in 2015, although they did divert some resources and a newly-acquired team to work on multiplayer shooter Fallout 76, which was released in 2018 and has recently seen a major new expansion released. Relatively little is known about Starfield apart from a very short video showing a space station floating in space. Leaks of varying degrees of reliability suggest that Starfield will be an open-world, space opera game consisting of a space station hub, spacecraft and several different regions on several planets the player can move between (possibly similar to Obsidian's recent SF CRPG The Outer Worlds, although Starfield is expected to be much larger). More fanciful leaks suggest that Starfield is set hundreds of years in the future of the Fallout setting, although this seems less likely.
Bethesda have hinted at announcements later this year to lay out their plans moving forwards. Bethesda's preferred release schedule is to fully unveil their game in the spring or summer and then release the game in the autumn, ahead of the Christmas rush: Fallout 4 was announced in June 2015 and released in November that year, whilst Fallout 76 was announced in May 2018 and released (at least in beta) in October that year. A surprise 2020 release for Starfield is therefore possible, although a 2021 release may be more realistic; even Bethesda might be wary of releasing a new game close to the release of CD Projekt Red's Cyberpunk 2077, another SF CRPG and widely anticipated to be the biggest game of the year, in September. Bethesda have ruled out any major new announcements in June, which suggests that if the game is to be fully unveiled, it will be much later in the year, making a 2021 release far more likely.
After Starfield, Bethesda's next project will be The Elder Scrolls VI, the eagerly-awaited sequel to their all-conquering 2011 game Skyrim. It is likely that that game will not follow until at least 4-5 years after Starfield.