"Spot on" means very accurate or correct, when you might not have expected it to be so accurate. For example, it's very difficult for an actor to play a real-life person from the past. However, Evan Peters's portrayal of Jeffrey Dahmer was spot-on: his acting and portrayal was extremely accurate. You may have friends or family who can do impressions of other people. If their impressions are accurate, you can say they are "spot-on."
A prediction can be spot-on, too. Before the government releases inflation figures every month, many economists predict the value. Often, these values are not exactly correct, but they are in the right neighborhood. However, sometimes an economist's prediction is spot-on: sometimes, that prediction is exactly the number that the government later releases.