Let's plot the point:



Now let's draw the horizontal line through it:



Now let's plot another arbitrary points on that line:



The arbitrary point on that line I chose is (2,5).

Question:  What do those two points have in common?
Answer:    They have the same y-coordinate, which is 5.

Let's plot another point on that line:



The arbitrary point on that line I chose this time is (-8,5).

Question:  What do those three points all have in common?
Answer:    They all three have the same y-coordinate, which is 5.

Do you see that no matter how many arbitrary points I pick on that line,
every one of them will have the same y-coordinate 5?

Since that is true for every point on that line, the equation of that line
could be the sentence:

  "The y-coordinate of every point on that line equals 5"

But we just shorten it to simply

           y = 5

That's the equation of that horizontal line.

Similarly every horizontal line has a very simple equation  

"y = some number"

Edwin