Algebra refers to the branch of mathematics where-in we use alphabets and numbers in formulas and equations to represent quantities that are otherwise unknown. For example, if one pen costs 2 \- then using algebra we can say that x pens would cost 2x. Here x would represent the variable that represents the number of pens. So if x = 10 then the cost would be 2 $\times$ x = 2 $\times$ 10 = $20/-.
Algebra in high school or college is usually split into 3 parts. They are: pre-algebra, algebra 1 and algebra 2. Thus, algebra 2 is the third algebra course in high school.