On-site generation places an electricity source close to its main user. A data center might use a generator, solar installation, or another source as part of its power plan. The equipment can supplement public-grid supply or support a more independent arrangement.
Producing power locally can change how much electricity a facility needs to import, but it does not remove the need to consider fuel, maintenance, emissions, cost, and backup. The best arrangement depends on the site's demand and the capabilities of the surrounding grid.
ELI5
On-site generation means making electricity at the place that needs it, or very close by. The site may still use the public grid too.
For example, a data center could run a generator on its property to supply some of the electricity its computers need. It would still need to plan for times when that generator is unavailable.
