If a straight line has a gradient (slope) of 0, it means the line is horizontal. The equation of a horizontal line only has a constant $y$-value.

Given that the $y$-intercept is 5, the line crosses the $y$-axis at $y = 5$.

Thus, the equation of the line is:

$y = 5$

Would you like further details, or do you have any questions?

Here are some related questions to consider:

1. How would the equation change if the gradient were non-zero?
2. What would be the equation of a line with a gradient of 0 and a $y$-intercept of -3?
3. How can you determine if two lines are parallel based on their gradients?
4. What is the equation of a vertical line passing through $x = 2$?
5. How does the $y$-intercept affect the position of the line on a graph?

Tip: A line with a gradient of 0 always has an equation in the form $y = c$, where $c$ is a constant.