In my book, the definition of the slope of the straight line is:
The slope is a measure of the direction of the line.
1) When the line has no slope, it tells that it is vertical or moving vertically along the $y$ and $y'$ axis.
2) When the slope value is equal to $0$ it tells that the line is moving horizontally along $x$ and $x'$ axis.
3) When the slope value is positive, it tells that the line is rising to the right.
4) When the slope value is negative, it tells that the line goes downward to the right.
5) A large positive slope value tells that the line goes along the $y-axis$ and is rising steeply to the right, and a small positive slope value tells that the line goes along the $x-axis$ and is rising slowly to the right.
Well, I'm not sure about the fifth one. Do a large positive slope value and a small positive slope value judge the direction of the straight line or if its rising steeply or slowly, does that judge the direction of the line?
Both:
You can use slope to determine how steep a line is; if a line is steep, it's slope will be larger than a line that is less steep, and the steeper the line, the larger its slope.
So they are mutually correlated: steepness increases as slope increases (directly and positively proportional): (each gives information about the other...
But steep, as a description itself, is relative to some orientation. Usually we mean that steepness is a measure of the absolute value of the slope: the larger the magnitude of the slope, the closer a line with that slope is to the y-axis.
The sign of the slope tells us in what in what direction the line is tilted, if it is "tilted" whether y is increasing from "left to right" (positive), whether $y$ is decreasing from left to right, or neither(0 = horizontal, or slope is not defined = vertical.)