What does @RC mean in SSDT SQL Server Unit Testing?
@RC = Return code. The boilerplate unit test code will assign the stored procedure return code value to this variable, where it can later be used in a test case assertion.

Assuming there's a subsequent SELECT @RC after the proc execution in the unit test code, the scalarValueCondition1 assertion in your screen image will verify the return code value is 1 in order for the test to succeed.