The clue is in the propagation speed. In free space a line with le = 1m would introduce a delay of td = 1/c0 = 3.333ns.
When, due to dielectric material (as in a coaxial transmission line), the propagation speed is less (about 0.66*c0), this line (with physical length of 1m) would show a delay of.
Td = 1/(0.66*c0) = 5.05ns.
This is more, so electrically spoken; this line appears to be longer.
Based on le = td*c0, the dielectrically filled line has an electrical length of:
el.le=td*c0 = 5.05n*3e8 = 1.52m.
So electrical length is the physical length of a line with propagation speed c0 that behaves the same as the actual line.
Note that unterminated lines have somewhat more electrical length, even when corrected for the propagation speed of the actual line. This is because of the capacitive fringing at the unterminated end.
If you express the electrical length in wavelength, you can say
electrical length = (physical length)/(wavelength IN trans.line)
Also here the line may appear longer due to fringe effects.