In SEO terms, Stop Words are words in the English language that break up a sentence into significant and less significant parts. Search engines will ignore Stop Words in a sentence and take up valuable space with no SEO benefit. To maximize your SEO efforts, it is especially important to avoid using Stop Words in Image Title, URLs, and Image Alt Tags. For example, take following Page Title for a fake site selling backpacks:

The best, most durable ultralight backpacks made in the USA.

This Page Title is riddled with Stop Words. Now consider the following example: > Ultralight backpacks - Made in the USA.

This version is short, sweet, and to the point. It lacks the same Stop Words found in the first example and contains the most significant topic related words at the beginning of the sentence. Although there is no official list, a list of English Stop Words is maintained here.