Again, a guess, but this reads like Horatio is being told that he does not know everything. If I could read prior passages of this statement the person might even be trying to tell Horatio that he has been brought up a certain way which prevents him from realizing how vast this world is.
Translate - There Are More Things In Heaven And Earth Horatio Than Are Dreamt Of In Your Philosophy.
Hamlet told Horatio that he doesn't know everything.
I think that it means that horatio's opinion isn't the only one that matters and he does not know everything, and sometimes he should listen to other people