For Anglophones, "classics" invokes the ancient Greeks and Romans. In literature, a classic is a particularly influential text. In science and engineering, a classical technique or model is one that is powerful but simple enough to be taught to freshmen.
A classic is timeless, enduring and leaving a wake. Its maturity is revealed by age, esteem, effect, and complexity.
Example. Newton's Principia, a work published 335 years ago, is still the foundation for physicists. But even modern quantum mechanics relies on Lagrangian and Hamiltonian mechanics, which are mathematical reformulations of Newton's work. Going back to fundamentals can be fruitful.
Example. The Greeks and Romans, for better or for worse, greatly affected the development of the Western world, many times over. To get a good grasp on the most impactful works of literature and philosophy, it helps to start at the beginning. If you need a starter, try this. At some point, though, you should outgrow it and have your own canon.
Read the classics. Or read Italo Calvino's Why Read the Classics? Not for Truth but to know where we came from and how far we've come. And then go further. Derek Walcott writes in Sea Grapes: "The classics can console. But not enough."