It was one starting QB for another, Jared Goff for Matthew Stafford.

Four-plus years later, it has aged in a fascinating way.
Jared Goff-Matthew Stafford trade details
The trade went down in January 2021:
Rams received
- QB Matthew Stafford
Lions received
- QB Jared Goff
- 2022 first-round pick
- 2023 first-round pick
- 2021 third-round pick
Detroit turned those picks into Jahmyr Gibbs, Sam LaPorta, Jameson Williams and a few useful defenders via a few additional trades.
Why did the Lions trade Matthew Stafford for Jared Goff?

The Lions had decided they needed to rebuild, and getting the draft picks they got for Stafford certainly helped them do that.
It still meant giving up on a franchise legend.
Goff has grown into a better QB in his time in Detroit, so that part of the deal has worked out fine, and to end up with Gibbs, LaPorta and Williams out of it too is a big win.
Stafford, of course, led the Rams to a Super Bowl in his first season. There’s no replacing that.