Similar to the Tone Page, the Pinyin page is like an index or database for any possible pinyin spelling. At the top, you will notice "Initial" and "Final". They indicate the part of the pinyin search option that you are entering. In this case, the letters below represent the initial part of any possible pinyin.
For instance, if we enter "t" as the initial part of the pinyin and select "ou" as the last part of the pinyin spelling, we get multiple examples using the pinyin "tou". The grayed out buttons indicate that there is no pinyin that uses that spelling in combination with the first initial.