keyword search is matching words. The existing of the query words within the search space items considered a match. A more advanced form is counting the number of occurrence of the query words within the search space item. (exact matching, or counting occurrences)
Keyword searching still has limited functionality compared to the broader capabilities the semantic search supports.
semantic search is more powerful, you are not matching words but meaning. It can match 2 different ideas that are not sharing the same words but share the same meaning. (works on the level of meanings)
Example:
search query: apple sentences: I like fruit salad
The keyword says that there’s no relation between the query and the sentence, while the semantics search at the level of semantics says that “apple is a fruit” so there’s a match.
semantic search can even match a word like “apple” with an image of apples, a video contains apples, fruits or a sound track talking about plants and fruits. while keyword search won’t.