There are different interpretations of what foods it allows. The general principle behind it is that food should be natural. Rastafarians therefore avoid food with artificial additives (e.g. colourings, flavourings, and preservatives - even salt), or which is chemically modified.
In common with other religions which use Old Testament teachings as a basis for their beliefs, such as Judaism and Islam, Rastafarianism prohibits the eating of pork. Most Rastafarians take this further, and consider the I-tal diet to forbid the consumption of all red meat (i.e. the flesh of mammals), although fish is sometimes allowed. Some Rastafarians go even further than that, and become strict vegetarians. At the other extreme, some Rastafarians do not adhere strictly to the I-tal diet at all.
The word I-tal derives from the English word vital, with the intial syllable replaced by the word I. This is done to many words in the Rastafarian vocabulary, to signify the unity of the speaker with all of nature.