Plug in values of x to find corresponding values of y.
For example, replace x with 0 to find that...
y = -5x-3
y = -5*0 - 3
y = 0 - 3
y = -3
We have x = 0 lead to y = -3.
Therefore, the point (0, -3) is on this line. This is the y intercept.
Now try something like x = 1
y = -5x-3
y = -5*1 - 3
y = -5 - 3
y = -8
It shows the point (1,-8) is also on the line.
The straight line goes through (0,-3) and (1,-8)
We can repeat this process infinitely many times since there are infinitely many numbers to pick for x.
See the graph below. I used GeoGebra to make it, but you can use something like Desmos or similar.