Straight lines are horizontal if they go across from side to side, without going up or down at all.
Straight lines are vertical if they go straight up and down, or top to bottom, without going across at all.
Lines are slanting if they don't go straight across or straight up. They look like a slope and go both up and down, and across too.
Look at this picture.
How many horizontal lines are there?
There are 6 shown in red below.