"That is a lot of meat."
Translation:Dat is veel vlees.
The modern model of grammar assigns veel to be a determiner, rather than to be an adjective, since it refers to the number of its subject, rather than modifying its size, color, type, etc.
Vele is the definite form and is used when referring to a specific lot, which is typically accomplished by using a definite article. Veel is the indefinite form, and is applicable here.
I personally prefer to think of veel, weinig, and such as being determiners rather than adjectives, because no exception to the rules of inflection/declension are then required. Enough exceptions!