A marginal seat is a parliamentary seat which the incumbent holds after achieving a relatively small majority in the previous election. A small swing in votes from one party to another in a marginal seat would make it likely that the seat would be lost by the incumbent. Marginal seats are perceived as the most significant to the result of a General Election, since most parliamentary seats are "safe".
Great resource for facilitating learning on electoral reform
4th November 2021
The German elections and the system for electing MPs
29th September 2021