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"