A marginal seat or swing seat is a constituency held with a small majority in a legislative election, generally one conducted under a single-winner voting system. In Canada, they may be known as target ridings. The opposite is a safe seat.
These seats only require a small swing to change hands and therefore are typically the focus of most campaign resources. The concentration of money and human resources in areas where they will make the most difference is known as targeting.
Political parties often face tension between the holders of marginal seats and safe seats. Safe seats tend to be allocated far less discretionary resources--governmental as well as political--from their political party than do marginal seats.
A similar phenomenon happens in United States presidential elections, where the Electoral College system means that candidates must win states rather than the popular vote. Again, resources are concentrated towards the swing states with the smallest majorities.