A feather in your cap is an achievement you can be proud of.
For example, "It will be a feather in Jenny's cap if she can retain the gold medal."

「a feather in your cap」指的就是一項你可以引以為傲的成就。
例如：「如果Jenny可以衛冕這面金牌，那會是一件值得驕傲的事」。