Coulomb barrier is the electrostatic repulsion between positively charged atomic nuclei that must be overcome before they can approach closely enough for the strong nuclear force to fuse them.
The barrier takes its name from Charles-Augustin de Coulomb, whose inverse-square law describes the repulsive force between like charges. Two nuclei feel that repulsion grow as they approach, until at separations of a few femtometers the strong nuclear force — powerful but short-ranged — takes over and binds them. For a 📝deuterium and a 📝tritium nucleus the barrier peaks at several hundred kiloelectronvolts, far more energy than even a 100-million-degree plasma gives its average particle.
Fusion happens anyway because of quantum tunneling: a nucleus has a small probability of passing through the barrier rather than over it, a mechanism George Gamow first applied to nuclear barriers in 1928. Tunneling probability rises steeply with collision energy, which is why fusion favors the fast ions in the high-energy tail of a plasma's thermal distribution, and why the 📝D-T reaction — with the lowest effective barrier of any practical fuel pairing — is the reaction of first-generation power plants. The barrier is also the reason 📝fusion needs temperatures above 100 million degrees Celsius at all: heat is simply the means of giving nuclei enough closing speed to make tunneling likely, in matter that at such temperatures exists only as 📝plasma.