Wire resistance and voltage drop
A wire is a pipe for electrons: longer pipes and narrower pipes resist the flow more, and the pressure you lose along the way is the voltage drop.
R = ρ × L / A ρ(copper) = 1.72 × 10⁻⁸ Ω·m
Annealed copper at 20 °C; resistance rises about 0.4% per °C. Aluminium is about 1.6 times worse. Keep the drop under 3–5% of the supply for motors and servos; thin breadboard jumpers are often the real cause of a resetting microcontroller.