From Academic Kids
A marginal seat is a district or constituency held with a particularly small majority in a Parliamentary election conducted under a non-proportional electoral system. These seats require a smaller swing to change hands and are therefore typically the focus of most of the parties' campaign resources. The concentration of money and manpower to areas where they will make the most difference is known as targeting.
Political parties face inherent tension between the holders of marginal seats and safe seats. Holders of safe seats tend to get far less discretionary resources--governmental as well as political-- from their political party than do holders of marginal seats.
A similar phenomenon happens in US Presidential elections, where the electoral college system means that candidates must win states rather than votes. Again, resources are concentrated towards the swing states with the smallest majorities.