Glitch is a short duration pulse or spike that appears in the outputs of a ripple counter with number<$2_n$.
Consider the waveform of a MOD-3 ripple counter shown below. But in practice, at the third falling clock edge, $Q_B$ and $Q_A$ become 11 causing a pulse.
Hence output of reset logic goes low but after a short duration of time. Here 11 is an unwanted state. In the output waveform of $Q_A$, this short pulse is called a 'Glitch'.
The state diagram of glitch is shown below. We can see that the state machine temporarily goes into unwanted state 11 which causes glitching
The waveform of this situation is shown below.