Full Idea
By 'form' Bell means the relations of lines, colours and shapes. Forms are 'significant' when the relationships of lines and so on move us aesthetically. If something is art it must have, to at least a minimum extent, significant form.
Gist of Idea
'Form' is visual relations, and it is 'significant' if it moves us aesthetically; art needs both
Source
report of Clive Bell (Art [1913], p.17) by Susan Feagin - Roger Fry and Clive Bell 3
Book Reference
'Key Thinkers in Aesthetics', ed/tr. Giovannelli,Alessandro [Continuum 2012], p.119
A Reaction
So art has two necessary conditions - that it move us aesthetically, and that it does so by means of its form. The obvious problem is to explain which forms are 'significant' without mentioning the aesthetic feeling they have to invoke.