Spikes on a square wave of a oscilloscope reading
That's called ringing, and it's an effect of inductance and capacitance in the circuit (often just parasitic).  Picture it this way...the wire from the driver to the receiver has intrinsic inductance, and most digital inputs have [gate] capacitance.

Once the output starts driving the line, current builds up in the (inductive) trace until the capacitance fills up to the same voltage as the driver.  There's still a current flowing through that inductance, though, so it continues pumping the capacitor higher until the current stops...now the voltage drives the current back to the driver.  That's the first peak, and the next time the current stops is the first valley.  Back and forth until resistance dissipates the stored energy.

Of course, the trace is actually a transmission line with distributed inductance AND capacitance, as well as a speed of light limitation, but that's about half a semester worth.

This can affect the signal integrity of the circuit when the ringing is large enough, as well as causing permanent damage to ICs over months or years of repetitive overvoltage/undervoltage.

Of course, the scope probe becomes a part of the circuit when applied, and can change the behaviour of the node.