Concept-based search can returns hits on documents that are "about" the subject/theme you're exploring, even if the words in the document don't precisely match the words you enter into the query.
Excite is currently the best-known general-purpose search engine site on the Web that relies on concept-based searching. and clustering -- which essentially means that words are examined in relation to other words found nearby.
For example, the word heart, when used in the medical/health context, would be likely to appear with such words as coronary, artery, lung, stroke, cholesterol, pump, blood, attack, and arteriosclerosis.
If the word heart appears in a document with others words such as flowers, candy, love, passion, and valentine, a very different context is established, and the search engine returns hits on the subject of romance.
Warning: This often works better in theory than in practice. Concept-based indexing is a good idea, but it's far from perfect.
Results are best when you enter a lot of words, all of which roughly refer to the concept you're seeking information about.