The term "seat" has several meanings, depending on its context.
1) In a general sense, a seat refers to any place where someone can sit or be seated - such as in a chair, bench, or car. For example, you might say that there are four seats around the dinner table.
2) Politically speaking, a "seat" is an electoral district represented by one member of parliament or congress. So when people talk about gaining or losing "seats," they mean winning or losing elections in specific districts.
3) In transportation, specifically referring to vehicles, a seat refers to the space provided for passengers. For example, cars and buses have different numbers of seats depending on their size and design.