Can you explain this to me please?

sin2 A + cos2 A = 1 ... (1)
1 + tan2 A = sec2 A ... (2)
1+ cot2 A = cosec2 A ... (3)

They are trigonomewric identities that are always true, not matter what the angle A is. All can be derived from the Pythagorean theorem of a right angle triangle.

If you do not know what sin, cos, tan, sec and cosec mean, I suggest your review your introductory reading materials, and http://www.mathwords.com/t/trig_identities.htm