“I sent in an absentee ballot last week.”
You're talking with a friend about voting. You voted before the election. You explain to your friend how you did that.
I sent in an absentee ballot last week.
To "send in" something means to submit it by mail or email. It's similar to the word "submit", but "submit" can also be used to describe giving something to a person by hand, whereas "send in" cannot.
You "send in" a résumé, an application, a payment, or some other official document. You submit it to someone who has authority.
This phrase can be ordered two ways: "send ___ in" or "send in ___":
Have you sent in your application yet?
I sent my rent check in last week.
But always put "it" and "that" before "in":
I can't remember if I sent it in.
A "ballot" is the form that a person fills out when voting in an election. Ballots can be printed on paper, or now they can also be electronic.
An "absentee ballot" is a ballot that you fill out when you're not able to vote on the normal election day. In some countries, you can request to mail in an absentee ballot before the election instead of showing up on election day.
The word "absentee" comese from "absent", which means "not there".