The grammar pattern 〜ついでに is used to express doing something "on the occasion of" or "while doing" something else, typically when you take the opportunity to do something extra while you're already engaged in another activity.
It conveys the idea of doing something additional or incidental while performing the main action. In English, it can be translated as "while you're at it," "on the way to," or "since you're doing X, you might as well do Y."
Verb (dictionary form) + ついでに
例: 郵便局に行くついでに、スーパーにも寄った。
("While I was going to the post office, I also stopped by the supermarket.")
Verb (た-form) + ついでに
例: 買い物したついでに、新しい靴も見てきた。
("Since I went shopping, I also checked out new shoes.")
Noun + の + ついでに
例: 散歩のついでに、カフェに寄った。
("While I was out for a walk, I stopped by a café.")
In summary, 〜ついでに is used to describe taking advantage of the opportunity of doing one action to do another thing as well, often with the second action being somewhat incidental.