In classification you usually have a single correct class, a hard label. However, you might also have soft labels, where multiple classes have non-zero target probabilities. Label smoothing is a technique that artificially introduces those soft labels from hard labels, i.e. if your hard label was [0 0 1 0] it might now be [0.05 0.05 0.85 0.05]. You could use the strength of smoothing to represent uncertainty.