A returning officer is someone who helps to make sure that an election goes smoothly. Think of it like someone who helps to make sure your class election at school is fair and everyone has a chance to vote.
The returning officer is in charge of things like making sure all the people who are allowed to vote are on a list, finding a place for people to come and vote, making sure people can vote in secret, counting all the votes when everyone is finished voting, and announcing who won the election.
So basically, the returning officer helps to run the election and make sure it is fair and everyone has a chance to have their voice heard.