By titling her novel Silas Marner, Eliot is participating in a long tradition of naming books after their protagonist: Daniel Defoe's Robinson Crusoe, Samuel Richardson's Pamela, Jane Austen's Emma, Charlotte Bronte's Jane Eyre, and so on. But there are two rather more interesting things to note about Silas Marner.
First, a lot of the book isn't really about Silas Marner at all. The narrator spends a lot of time on Godfrey's story, and quite a bit of time on the village and villagers. This tension between the individual (Silas) and the community (Raveloe) gets at exactly one of the novel's main interests: what is the relationship between an individual and his neighbors?
And (second) that's where the subtitle comes in. The subtitle takes away from the focus on the individual by locating Silas in a community and giving him a role. He's not just "Silas": he's a weaver (he has a function in the village) and he belongs to a specific group of people. As you read from left to right, you almost get the whole story of Silas Marner: Silas starts out as an individual "Silas Marner"; he then defines himself through his work ("the Weaver"); and then he finally assumes a role in his community ("of Raveloe").