The most interesting new hep-th arXiv paper today is the DESY-Rutgers-Beijing work

by Elli Pomoni, Yan, and Zhang. Instantons are localized configurations in the Euclidean spacetime (they are also localized in the Euclidean time, in this sense they live at an "instant", and that explains their names) and as local extrema of the path integral, they contribute to the transition amplitudes in Feynman's approach to quantum mechanics. The contribution is typically nonperturbative at small \(g\), scaling like \(\exp(-C/g^2)\) at least in the simplest cases.Tetrahedron Instantons

In gauge theory, we have the simplest or "real" instantons in which the field strength is self-dual but arranged in a topologically nontrivial configuration that makes its magnitude decrease away from the center of the instanton. In string theory, there are other instantons that may be made of strings and branes, not just fields (gauge fields).