While Presidential elections get the most attention, "midterm" elections—held every four years in the middle of a President's term—are just as important for the direction of the country.
In a midterm year, voters choose:
Midterms decide which party controls Congress. If the President's party loses control, it becomes much harder for the President to pass their agenda. Midterms are often seen as a "referendum" or a grade on the President's performance so far.
Historically, fewer people vote in midterm elections than in Presidential elections. This means that those who *do* show up have an outsized impact on the outcome and the future of the government.