Berkeley is no doubt an idealist, a person who believes only on the mind, there is no such thing as matter according to them!
Berkeley believes that ideas from the mind can come from the senses, remembering/manipulating, or combining ideas (you combine ideas to have an idea of a mermaid for example).
Lets say you have an apple in front of you, a dualist, you think that the apple consists of matter, and the idea of it comes from the mind, thus the mind and matter both exist independently. However Berkeley way of thinking goes like such: an apple for example is just a cluster of ideas of it being red, sweet, round...and thats all the apple is; a cluster of ideas you perceive. Thus he believes that an apple or anything for that matter (mountains, rivers,) would never exist without a mind to perceive those ideas.
Ultimately according to Berkeley ideas can only come from the mind, or god but never from matter itself.
Berkeley believes that the apple has secondary qualities like color and it also has primary qualities like shape, figure. He thinks the primary qualities of anything can only be known from the secondary qualities, thus both these types of qualities are mind dependent, and matter independent.
To wrap up, according to Berkeley no matter exists, only ideas that are perceived by the mind exist.
Myself I don't think i am in any type of agreement with Berkeley's way of thinking, it seems more logical and natural to believe in a type of dualist approach where there can be both material objects and the mind working independently.