Because we associate berries with being bitter and sweet, they act as a perfect symbol for bitter sweetness. If you are not sure what bitter sweet means outside of the context of flavor, this will attempt to explain it. And just remember that this dream could also be an indication that you are going through a bittersweet period currently and does not have to be a prediction of the future necessarily.
As stated above, dreaming of berries suggests that you will be going through a bittersweet experience, and if you are unsure of what that means, it is basically something that is both good and bad in equal parts. This is not to say that something good will happen immediately followed by something bad. It is more of a mélange of good and bad and is really more complicated than that. It is the type of thing where you undertake a situation that will have both negative and positive effects in the long run, effects which cannot easily be measured against one another to choose what would be the best outcome, it is not that simple.
For a simple example, imagine a friend invites you to go bowling with him or her and you tell them yes. Just afterwards you get a call from a friend asking if you want to spend time with them except this friend is one that you haven’t spent much time with recently and really misses you. On the one hand it would be great to go to the bowling alley and have a fun time with your friends, but the other friend would be left alone, and it might affect your enjoyment of the bowling. On the other hand if you choose to go spend time with the friend at his or her house, then your friends that you promised you would go bowling with will be hurt or insulted by your choice and you will have to carry that on your shoulders the rest of the night and until the incident is forgotten totally.
Another example that is more generalized of a bittersweet situation is one in which you have to give up something important to you in order to achieve a greater good that you want even more. Dealing with sacrifice in the face of an overall greater joy is the essence of the meaning behind this word. This is what it means if you eat berries in your dreams. Prepare to deal with something bittersweet later on.
Berries also suggest when you eat them in your dreams, that you will be having a nice and fulfilling relationship with someone in the future. If the berries are poisonous or they are too sour then this implies that a relationship you have with someone will become a negative influence on your life.