Welcome to the Forum and congratulations on your new firearm
As has alredy been mentioned, that is one edge of the side plate. If you follow that seam around, you will notice that two of the three screws that hold the side plate in place are visible inside the boundary of that line
The side plate removal allows for access to the innards of the revolver. Pictured here is my Model 66 with a clear polymer side plate installed so that you can see what is "Under the Hood"