Can someone please solve Euclid's proof of the Pythagorean Theorem?

I understand you are supposed to get the triangles congruent by SAS Postulate. Then, I'm confused on how to the triangle half the area of the squares. On the website, http://www.cut-the-knot.org/pythagoras/index.shtml, it says "ΔABF has base AF and the altitude from B equal to AC. Its area therefore equals half that of square on the side AC. On the other hand, ΔAEC has AE and the altitude from C equal to AM, where M is the point of intersection of AB with the line CL parallel to AE. Thus the area of ΔAEC equals half that of the rectangle AELM. Which says that the area AC² of the square on side AC equals the area of the rectangle AELM"...can somebody clearly explain what this means? In the beginning, it says to "ΔABF has base AF and the altitude from B equal to AC."...what altitude? It's so confusing. Do I plug in numbers? Help Please!!!