The term circumstantial evidence refers to evidence that does not directly prove that a defendant has committed a crime.  In this case, it would be evidence that suggests that Adams committed the murder but does not directly prove it.

Technically, though, Greene is using this term incorrectly.  All the evidence that he says will be given is circumstantial.  People have not seen Adams actually killing the victim.  That would be direct evidence.  Instead, Mrs. Salmon has, for example, seen Adams come out wearing gloves and looking scared.  She has then seen him drop the murder weapon in some bushes.

This is circumstantial evidence because it only implies he is guilty.  Salmon did not see him actuall kill the victim -- she has just seen him act in ways that suggest he did.