1. Please tell me about what is calculus For what it's used I am a beginner.
Wow, THAT is one big, big, big question.
Here’s a curve.
Here, the slope is much steeper than at the bottom
At the bottom, the its slope is horizontal, or flat.

2. Here, the slope is between the previous two.
So the slope of this curve, or ANY OTHER CURVE varies depending on where you are on the curve.
You can MEASURE the slope of the curve at any point by drawing a tangent (the straight lines), then drawing a right angled triangle, then calculating the tangent to the angle shown.
What CALCULUS (inventor, UK scientist Sir Isaac Newton) does is to find a way of being able to CALCULATE or work out the slope of the curve at any point, using a FORMULA which can be derived, once you know the principles.

3. The very rough sketch above is meant to be that of a PARABOLA, and I’m going to use a formula for a parabola ...
Once you’ve waded through all the algebra, it turns out that the formula which gives the slope at any point on this parabola is
That “wading through” bit is another way of saying DIFFERENTIATING the equation of the curve, and the ideas of how to DIFFERENTIATE algebraic expressions and formula is very important in calculus.
You can check this lot. If you draw a PARABOLA, and then select some points on it, you can MEASURE the slope at those points, using a ruler, pen, and whatever other bits you fancy.
If you compare your measurements to CALCULATIONS using the formula for the slope AT THE SAME POINTS, then they should agree.