As others said, exit is where you jump from. Exits matter because not all are safe vertical drops, some are way more stylistic than others and in case of BASE wingsuiting, some lead to specific lines. A line is the flight path a wingsuit pilot takes after jumping. Recent times, proximity flying has been getting more popular, where a person wearing a ram air wingsuit tries to get as close to cliffs, ground, trees or other natural elements as possible. As you can guess, lots of people die doing this.

For an example of a safe exit that immediately leads to a very spicy line, check this video, at 0:40 specifically: https://vimeo.com/82949889