Keep one line fixed, and rotate the other with a slider. Watch how the angle where the two lines meet decides whether they're parallel or perpendicular.
Two parallel lines never meet, no matter how far you extend them. Perpendicular means the angle where two lines meet is exactly 90ยฐ (a right angle). Any other case is just called "meeting" or "intersecting."
You can spot these all around you โ the two side rails of a ladder, or the two rails of a train track, are parallel. The horizontal and vertical edges of a book or a window frame are perpendicular. The two hands of a clock pointing to 3 and 12 are exactly perpendicular too.
Move the angle slider bit by bit, and find the exact moment the two lines become parallel, and the exact moment they become perpendicular. As the angle gets close to 0ยฐ or 180ยฐ, the two lines look almost like one line. As it gets close to 90ยฐ, you'll see a neat cross shape form.