When applying a signal to a tline, since the tline has L and C, the tline will be charged, so there will be current flowing in. The square root of L/C is called characteristic impedance, the exact moment when it charges, it looks like a resistor which has a resistance of the tline's characteristic impedance.

Since it charges, energy flows into it. Since EM wave propagates at a specific velocity, when it reached an end which is not properly terminated, the exact charging current can mot be perfectly transfer to the load, so the mismatched part reflects back at the same velocity.

The reflected wave superimposing with the transmission wave created standing wave, so at some point, there will be higher voltage, while at some points there will be lower voltage.

For a short enough tline, say, much shorter than wavelength, the standing wave is not obvious, so you can ignore it. When frequency increases, the standing wave is more and more noticeable. When the tline is long enough, the standing wave reaches its max value, where VSWR equals to the ratio of |Zin| and |Zout|.