What's the meaning of Electrical Inches?

"Electrical inches" is shorthand for "inches of trace along which the electrons must flow". In other words, it's the length of the wires or traces, following any bends, not the straight-line distance.

An electrical inch is the actual distance along an electrical path.

Source: UltraScale Architecture PCB Design from xilinx.com

Footnote on page 19
Referred to as "Added footnote with definition of electrical inch to 0805 Ceramic Capacitor" in revision history (version 1.6).