A schematic label whose name is visible across all sheets. Two pins anywhere with the same global label are the same net. The simplest connection mechanism in a single-sheet design.
How it looks
Real image: the CORE3588 schematic, exported by KiCad (shown in greys)
Labels on the CORE3588 schematic. These are ordinary (sheet-local) labels: CORE3588 has 623 of them and no global labels, because almost everything sits on one sheet. A global label works the same way, but across all sheets.
1"CAM0_SDA" is a net label: any wire with the same label on this sheet is the same net, with no line drawn between them.