The global sales of electrolyzers in 2017 were estimated to be 100 megawatts per year. That is enough to produce approximately 50,000 kilograms of hydrogen per day (assuming 50 kWh/kg, running full time). An electrolyzer is a device used in electrolysis, which is a process to produce hydrogen. During electrolysis, an electrolyzer uses electricity to split water molecules (or H2O) into hydrogen and oxygen. This is just one example of the multiple ways in which you can produce hydrogen.
Sources: E4tech, NOW, and Fraunhofer, Hydrogen and Fuel Cells Program Overview, 2018 Annual Merit Review and Peer Evaluation Meeting