Why can't I search for words with 3 characters or less?
This is a limitation of the MySQL Full Text search function.Find more information on the MySQL site.
Note that with XenForo 1.1 and above, it is now possible to set the minimum word length in the ACP -> Options -> Search Options: Search Minimum Word Length; this value must correspond with the MySQL full text minimum word length in the my.cnf configuration file.
Why can't I search for certain words?
The MySQL search function has what are called stopwords, which are typically common words, and are excluded from searches. Find more information on the MySQL site.
Note that the XenForo Enhanced Search add-on, which is installed here on XenForo.com, does not suffer the same constraints as default MySQL search does with regards to minimum word length and stop words. The stopwords for elasticsearch are listed here.