There is a lot of gas in the Universe, most of it hydrogen. The gas is denser in some parts than others, and densest in galaxies. If it is dense enough, or it is disturbed – for example by the shockwave from a supernova – then it can collapse, with the collapse proceeding quicker as it proceeds. As the cloud contracts it heats up. The collapse speeds up as it proceeds, and the gas heats up more and more... Eventually the gas at the centre because hot enough so that the nuclei – mostly protons/hydrogen nuclei – have enough energy to overcome their mutual repulsion, and fuse together into heavier nuclei. This process of light nuclei fusing to become heavier nuclei is called 'fusion'. Nuclear fusion generates a lot of heat, so the gas particles move faster and exert a pressure to stop the cloud collapsing. An equilibrium is established between gravity and gas pressure – a star is born.