You can check the definition of this English idiom here.
A feather in your cap is a success that will help your future.
A feather in your cap is some occurrence or accomplishment which distinguishes you. Imagine 100 people all wearing the exact same hat ( or cap) and one person in the group has a brightly colored feather protruding from his hat. Your attention is more likely to be drawn to him/her. So, a perfect score on a test, a job well done, receipt of some high honor etc is therefore your "feather".
There is an idiom in Turksh as the same meaning ''partridge in the bag''':it is easy for you,the conditions are in favor of you.