Feynman argues that useful scientific knowledge should do more than describe events that have already occurred. A theory gains value when it can be used to make predictions about what will happen under specified conditions. Those predictions can then be compared with observation, allowing the theory to be tested rather than merely used to explain the past.

The wider quotation also emphasises the risk involved in prediction. To say what will happen next is to make a claim that may turn out to be wrong, and Feynman treats that exposure to possible failure as an essential part of scientific inquiry. A theory that can accommodate any outcome after the event is difficult to test meaningfully. The quotation therefore links knowledge with predictive power, testability and a willingness to state expectations clearly enough for evidence to confirm or challenge them.

Full quotation

“Knowledge is of no real value if all you can tell me is what happened yesterday. It is necessary to tell what will happen tomorrow if you do something—not only necessary, but fun. Only you must be willing to stick your neck out.”