Meaning is a very slippery concept - there is a lot of ongoing debate among philosophers as to what it actually is. One relatively uncontroversial point that is raised often is that to understand anything requires consciousness, so to have a search engine that could actually "search by meaning" you would be looking at something based on some serious Artificial Intelligence. I don't know how far we have got in that direction as yet, but I don't think we have any artificial consciousnesses so for the time being meaning-based search is probably out.
The classic example of why there are problems with this is the difference between two similar phrases:
Time flies like an arrow. and
Fruit flies like a banana. Both are very similar from the perspective of language, yet someone who understands both will have used a lot of contextual knowledge to distinguish between them.
The avenues that are moving in the right direction are text analytics and data mining techniques, but they are still very much work in progress. You might want to take a look at Apache Mahout as a starting point for learning more about this.