What does Yale's motto, "Lux et Veritas" mean? Light and Truth I know, but what does THAT mean?

As the first answer suggested there IS a biblical origin to the expression written at the bottom of the Yale University seal. But the KEY to the SPECIFIC origin is found in the TOP part of the seal, where you will see a pair of HEBREW words written. אורים ותמים

In English letters, 'Urim we-Thummim, generally simply rendered "Urim and Thummim" in Bible translations.
http://content.answers.com/main/content/wp/en/thumb/9/9a/198px-Official_Yale_Shield.png

"Lux et veritas" ("light and truth") is meant as a TRANSLATION of this Hebrew expression.

So, a bit more on the Hebrew. The expression refers to some object or set of objects that were to be kept in the clothing of the high priest and used to consult the Lord God for answers to certain questions.

There is a great deal of mystery about exactly WHAT it/they looked like and HOW they were used to discern God's will. (A common view is that they were stones/lots, one meaning 'yes' another 'no'. But some of the apparent uses provide MORE than simple Yes/No or two alternative answers, so I'm not too sure about that.)

Here are the biblical passages that refer to them:

First - four passages in the Torah (five books of Moses), associating them with the high priest, esp. his clothing.
Ex 28:30, Lev 8:8, Num 27:21, Dt 33:8

Then there four narrative passages that mention them.
1 Sam 14:41, 28:6; Ezra 2:63=Neh 7:65

Unfortunately, only 1 Sam 14 gives us any clue of HOW they may have operated (and it does not look like "yes/no"). A few other passages which do not name them may, however, be recording instances of their use, e.g., Judges 1.

But the MAIN point is that they were a means of REVEALING God's will to his people (in certain instances).. AND the specific meaning of the Hebrew words.

Urim and Thummim, when translated, is generally taken as meaning something like "light(s) and perfection(s)" or "revelation and truth". (The FORM of each word is PLURAL, but sometimes such words are the so-called "plural of majesty", but have a SINGULAR sense. Anyway, you can see how the maker of the Yale seal took them.

This, of course, is a wonderful motto for a school (and note that Yale was founded to provide academic AND religious training, concerned first with the training of the CLERGY). There might possibly have been some other reasons for COMBINING the two terms, as the following article suggests, but I think the Hebrew expression itself best explains why it is a PAIR.

http://www.yalealumnimagazine.com/issues/01_03/seal.html

One other detail that probably played NO role in the Yale seal, but MAY explain somethign about the HEBREW expression. Urim begins with the Hebrew letter ALEPH, the first letter in the Hebrew alphabet; Thummim begins with TAU, the LAST letter. Thus it may be that the very names were meant to suggest knowledge "from A to Z" ("aleph to tau", compare Greek "Alpha and Omega"),,, that it was a means by which God made 'the whole truth' known to them. (This is the conclusion I came to studying the question some years ago -- the pattern strikes me as more than co-incidental. I do not recall anyone who specifically argued for it, though I expect some did.)

A bit more on the Hebrew background.
http://omega.cohums.ohio-state.edu/mailing_lists/CLA-L/Older/log94/9407/9407.26.html