Full Idea
Berkeley's thesis is that reality ultimately consists of a community of minds and their ideas; one of the minds (God) is infinite, and causes most of the ideas.
Gist of Idea
For Berkelely, reality is ideas and a community of minds, including God's
Source
report of George Berkeley (Three Dialogues of Hylas and Philonous [1713]) by A.C. Grayling - Russell Ch.2
Book Reference
Grayling,A.C.: 'Russell' [OUP 1996], p.25
A Reaction
I think Russell nicely pinpoints what is wrong with Berekely, which is that he confuses ideas with their contents. If I think about my garden, the garden is real (probably), which is the content, and they idea is just a way of thinking.