Why'd we have reflected waves in transmission lines?
When you say,

I can understand it if we talk about electromagnetics waves, it colides with a superficie and be reflected, but what happens in a electrical circuit?

You must understand that a signal propagating in a transmission line is propagating as an electromagnetic wave. Consider a coaxial transmission line for example. The signal propagates as a Transverse Electromagnetic wave (TEM wave) very much like a free space EM wave.
If you already understand that a free space EM wave will &quot;bounce off&quot; a surface, let me reframe the perspective so you can also understand why signals in a transmission line will reflect.
Any EM wave will reflect off of a discontinuity in the propogating structure's impedance. Whether that is an EM wave traveling through 377 Ohm free space reflecting off of a metal plate that presents a 0 Ohm (short circuit) impedance, OR that could be a TEM wave propagating down a coaxial cable with a characteristic impedance of 50 Ohms that then reflects off of a short circuit at the end of the cable.
Both cases demonstrate an EM wave reflecting due to what we call an impedance mismatch ie. an abrupt change or discontinuity in the  transmission line's impedance.
There is much more to learn about reflections and impedance mismatches such as the polarity of the reflected pulse, the exact ratio of the amplitude of the incident and reflected pulses, etc. However for the scope of this question I will stop here.