Semantic means the meaning of. It has to do with what things mean. So the semantic of something is the meaning of that thing.
To human eyes the meaning of a link on the web might be self explanatory. It might be obvious that that a link to “What is The Big Web FAQ?” is going to take you to a definition of this site.
To a computer however, it is just a bunch of letters in the same way that Chinese is just a bunch of symbols to an Englishman.
The Semantic Web is where relationships such as links, are defined in a way that a computer can understand. In the previous example the simple word “definition” would tell a computer that the link takes us to a definition of what this website is about.
More than just links, the Semantic Web describes the relationships between things such as time, weight, distance, definition, popularity. Anything really.