If two lines are subparallel, they are almost parallel but not quite.
Subparallel means the lines are very close to being parallel, but not exactly.
Examples:
The roads are subparallel, but one is a little bit higher than the other.
The two fences are subparallel, but one is a little bit longer than the other.
The two rivers are subparallel, but one is a little bit wider than the other.
The two lines are subparallel, like two friends who are almost the same height.
The two mountains are subparallel, but one is a little bit taller than the other.
The two walls are subparallel, but one is a little bit thicker than the other.