You should have an idea of the normal male and female reproductive tracts and the basic idea of sperm and egg.
There are roughly 4 stages in conception.
Men produce millions of sperms though only one sperm is necessary to fuse with the egg. Sperm production usually continues even beyond 40-45 years though numbers and quality may reduce and genetic defects are possible beyond paternal age of 40.
Women produce a fixed number of eggs throughout their reproductive period (~15-45 years). So it is very important not to waste the eggs during infertility treatment. All treatment on women work through induction of ovulation, i.e., increasing the egg production. This leads to reduced egg reserve and eggs may not be available when all other problems are sorted out.