how do you find the slope of the best fit line graph? And what is it supposed to tell us?

Since this is not my area of expertise, I searched Google under the key words "slope 'best fit line' graph" to get these possible sources:

http://www.physics.upenn.edu/~uglabs/Estimating-the-Best-Fit-Line.pdf
http://www.phy.ilstu.edu/slh/graph_errors.pdf
http://honolulu.hawaii.edu/distance/sci122/SciLab/L3/graphs.html
http://mtsu.edu/~phys2120/2121/Graph/graph.html
http://mathbits.com/MathBits/TISection/Statistics1/LineFit.htm

In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. For your second question, you might add "meaning" to the above key words.

I hope this helps. Thanks for asking.