I will explain the definition of the Kervaire-Milnor invariant of an immersed 2-sphere in a 4-manifold, which obstructs the sphere being homotopic to an embedding. I will describe two instances in 4-manifold classification where this invariant appears, one in the sphere embedding theorem and one in stable classification.
Links:
[1] https://www.mpim-bonn.mpg.de/taxonomy/term/39
[2] https://www.mpim-bonn.mpg.de/node/3444
[3] https://www.mpim-bonn.mpg.de/node/9096