"Very interesting" can literally mean something arouses interest.
HOWEVER, it is often used by English speakers to respond in a verbally neutral way, while also slightly masking or even hinting that something is displeasing. This is more common in the UK. For example, if at work, a colleague suggests a wild unfeasible project, and then asks his other colleague "so, what do you think?" The other colleague might say "very interesting..." to ostensibly remain neutral while still signalling his displeasure, but not needing to outright reject the idea and hurt his other colleague's feelings.