A good encyclopedia can give us its definition of glitch:
A glitch is a short-lived fault in a system.
The term is particularly common in the computing and electronics industries, and in circuit bending, as well as among players of video games, although it is applied to all types of systems including human organizations. The term derives from the German glitschen, meaning 'to slip.'
In electronics, a glitch is an electrical pulse of short duration that is usually the result
of a fault or design error, particularly in a digital circuit. For example, many electronic components such as flip-flops are triggered by a pulse that must not be shorter than a specified minimum duration, otherwise the component may malfunction.
A pulse shorter than the specified minimum is called a glitch. A related concept is the runt pulse, a pulse whose amplitude is smaller than the minimum level specified for correct operation, and a spike, a short pulse similar to glitch but often caused by ringing or crosstalk.
Content provided by Wikipedia.
In a few words, a glitch is an action on a game that is not supposed
to happen. The most popular type of glitch that exists is the Programming glitch, product of a badly programmed section in
the game. However, a “graphic mistake” is also a glitch, known, in other words, as a graphic glitch.
The Pokemon games are famous to have many glitches in them, specially the games of the first generation (Pokemon Red, Blue, Green and
Yellow). Games after these ones don’t have many glitches as the first ones, but they still have.
But why glitches happen? Because the programmers of the games are not
aware that these could happen. These flaws of the programmers made that the most famous glitch in the Pokemon games existed,
which is called, as many know, MissingNo.
Many, many glitches are present in the Pokemon games. There are so
many, that a web site is about these glitches. And that web site is this web site.
If you still don’t know what a glitch is, you can search its definition.