Translation:A doctor should have confidence.
Not while they practice!!-)
For those who are not aware, in American English, medical doctors are said to be in practice referring to their job. The 'usual' meaning (for most other professions/activities) of the word 'practice' implies 'working to improve/perfect' something (for example athletes or artists practice prior to an actual performance...) Hence the joke about doctors practicing all their lives... Happy New Year!