Filter Words are a common issue that affects title tag quality and SEO. What are Filter Words? They're words ignored by search engines due to their common use and offering very little value toward making a page appear to have valuable content. Some examples of filter words are a, the, best, all, name, not, and new. There a several hundred words that are generally considered to be filter words.
Why are they a problem? First, to conserve disk space and assist in search performance, stop words are filtered out of a search engine's database and not cataloged. Therefore, no matter how many times filter words appear in your text, they'll be completely ignored.
Second, and perhaps more important, is that these words take up valuable space in a page title and dilute the importance of the other words. Consider these two titles:
Acme Brothers Offers the Best Custard, Shakes and Burgers in Milwaukee
Acme Brothers Milwaukee Custard, Shakes, Burgers
The first example has 11 total words, 4 of which are stop words and one (Offers) that is of questionable value to searching. Contrast that with the second example, which has six total words, all of which are valuable to searching and pertinence to the page and the site. After removing the filter words, we're left with a title tag that's highly optimized.
One important point to note is that removing stop words typically results in title that appear stiff to a human reader. A compromise like "Acme Brothers - Milwaukee's Favorite Custard, Shakes and Burgers" creates a more excitement, only has two lesser-quality words, and fits in 64 characters. As a personal preference, I occasionally add filter words into titles that need to appear more natural or if I have many titles o a site that are stripped of filter words. That little bit of variety won't seriously impact page positioning but will make the titles more appealing and more likely to be clicked on.