The proton-proton chain is the name given to one common fusion reaction occurring in stars by which hydrogen is converted to helium, along with the release of energy.

Hydrogen, via the proton-proton chain reaction.

The proton-proton chain accounts for more than 98% of the fusion reactions in our Sun. The balance in the carbon-nitrogen-oxygen catalytic chain reaction.

Hydrogen-1 is fused into helium-4. This happens at high temperatures - somewhere around 14 million kelvin (which, at such high temperatures, is the same as 14 million degrees Celsius, for all practical purposes). There are two main reaction chains that in stars: the proton-proton chain reaction, and the CNO cycle. I suggest you read the corresponding Wikipedia articles, if you want more details about each of the reaction chains. Anyway, in our Sun, the predominant reaction chain is the proton-proton chain reaction.

Hydrogen (essentially a proton-proton reaction)