In general the definite article is used to indicate a precise (positive) thing, sometimes already mentioned in the same utterance, and to indicate its number:
"I cani abbaiano" (the dogs bark) means "all the dogs do bark" or, implicitly in a different context, "our dogs (out there) are barking".
"Cani abbaiano", without the article, or with an indeterminate one ("dei cani abbaiano") is a form more vague, uncertain. In an affirmative phrase is used to say that we hear barking, but we knows little more. If asking "ci sono cani che abbaiano?" the meaning could be "I am hearing something that seems as few dogs barking", or "is it possible that a dog barks? Does it exist even a single dog who barks?".
In your question, it happens that uva is a special term (also pointed out in comments), like "frutta" (fruits, but really "all the fruits, all the kinds of fruit"), "latte" (milk) and similar. Those nouns are not really countable, and the plural form is little used when it exists. They are indeterminate by nature (or, better, they are generic) even when the definite article is used.
So, asking about "cogliere uva" o "cogliere l'uva" is really the same, as would be with latte or frutta.
A little difference can be expressed in affirmative phrases: "io colgo uva" (I harvest grapes) is really generic, just to specify grapes instead of apples. "io colgo l'uva" can mean the same as before, or instead "I harvest my own grapes, after a year of work".
