Fusion means two nuclei fuse together (or you can say come together) to form a third, larger nucleus. Under special circumstances it is possible to push two nuclei together and make them fuse. When elements with low atomic numbers fuse, the mass of the final nucleus is less than the mass of its constituent parts. In these cases, it's possible to extract energy from the fusion reaction by conversion of the missing mass.

You may wonder how hydrogen bomb works. Hydrogen bomb first initializes a nuclear fission, and then use the energy from fission to start the nuclear fusion. Why doing this instead of starting the fusion directly? That is because fusion needs a lot more energy to start, and we can't make that much energy to start it except using a fission to generate that much energy to start the fusion.