It's an election held for a certain seat if the current person holding the position is unable to perform their duties any more - eg they die.
A by election is a type of election in which an election is held for one or two seats when a person earlier holding that seat dies or resigns!
A byelection is a small election held for one or two seats in a parliamentary legislature. In a general election, all seats are to be contested, but in the event that a member of parliament resigns or dies or for some other reason vacates the seat, they have a special election, a byelection, to fill that one seat.