Treadmills function by rotating a belt around a set of rollers.
To make the belt a continuous loop, it is connected to itself.
The process of joining the belt to itself leaves a visible "seal line", indicating where the belt has been joined together.
These lines are a perfectly normal part of your treadmill and are not a cause for concern, they will not impede the function of the treadmill.
If you have any other questions or concern about your treadmill, please contact our Customer Care Team, we will be happy to assist.