What does a yellow coloured Stage mean in Jenkins?

If a stage generates a junit test report which is processed by the junit step, and if the report contains test errors and failures, thuis is interpreted as a build ‘instability’ and the corresponding stage will be marked as UNSTABLE. The Jenkins classic view and Blue Ocean interfaces will render this as a yellow stage.

An unstable build stage reported in Jenkins' classic view interface

An unstable build stage reported in the Blue Ocean interface