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there are 2 types of lengths . one is physical length and other is electrical length.
if a wire is 1 metre long we can say its phsical length is 1 metre. now electrical length of wire will depend on the frequency of the signal which is passing through wire.if 300Mhz signal is passing through it then its wave length is 1 m.as wire lenth is also 1 meter so we can say electrical length of this wire is one wavelenght for this signal.however if you change the frerquency wavelength will change so will the electrical length.in short electrical length is always in stated in terms of wavelength.i hope this explanation should clear your confusion.
The intuitive undrestanding is like this:
when the signa is propegated thorurg the trnsmission line, when it travels once from source to load it may achive sufficient voltage level to de determined as high or low, beacause of impedance mismatch.
So the signal has to travel so some more time such that it will attain the sufficient volatge level. the distance it treavelled is electrical lenght....