In which figure is debc?
figure 1
figure 2
figure 3
figure 4

DE¯¯¯¯¯∣∣∣∣BC¯¯¯¯¯means that these sides are parallel and by going through the figure, figure 4 represents that DE is parallel to the BC because that these lines are straight which do not coincide with each other.

By using rules of mathematics, these side DE and BC are just scaled by 3. 5 to 15 and 6 to 18.  Parallel lines are paths in a level which do not coincide. This means that two lines in a plane that do not cross each other at any position.

By extension, a line and a plane, or two planes, in three-dimensional Euclidean space that do not share a point are said to be parallel.