When two vowels go walking, the first one does the talking.
A basic reading rule. When you come to a word that has two vowels together, the first vowel is usually the one you pronounce. It will also usually be pronounced as a long vowel, but we don't usually tell kids that -- we may say "it usually says its name" if we want to clarify.
Examples: Boat, mean, pour, wait, rain, please, speech, die, blue.
Exceptions: Quick, boil, boom, meant, beautiful, believe.
Obviously, this isn't a rule you can count on in all situations, but it holds true in enough words (particularity in common, one syllable words) that many educators find it useful. Other educators shun this rule, as it only holds true, overall, in about 50% of English words. Despite this it is still a favorite among special education teachers and reading tutors, although this may be simply because it makes reading more fun. This is actually not a rule you want to encourage kids to take to heart, it's just a useful reminder if you happen to be looking over their shoulder and see them having trouble.
There is also a catchy song or two that goes along with it; I have not been able to find the original source, but it may have been the PBS show Between the Lions. You can find various versions of the song on YouTube.
Silent vowels, both in "two vowels go walking" words and the 'silent e', are sometimes called vowel markers.
References:
http://coreknowledge.org/CK/about/CommonKnowledge/v20III_2007/v20_III_2007_vowels.htm Has lots to say on why this is a bad rule!
http://www.youtube.com/watch?v=75KAgF0NhHI&feature=related Has the Between the Lions song.
http://www.youtube.com/watch?v=bMvwhfQRIAc&feature=related has another version.
http://brendaclews.blogspot.com/2006/10/when-two-vowels-go-walking-first-one.html