What "Random Mutation" means in Evolutionary Biology

-
I don't know why this is so difficult but the "random" in "random mutations" just means they are strictly a chance, as in happenstance, occurrence. They are accidents, errors and mistakes.

The first step in selection, the production of genetic variation, is almost exclusively a chance phenomenon except that the nature of the changes at a given locus is strongly constrained. Chance plays an important role even at the second step, the process of elimination of the less fit individuals. Chance may be particularly important in the haphazard survival during periods of mass extinction.- Ernst Mayr "What Evolution Is"

That is it. Errors and mistakes happen during copying. Accidents occur when an organism is subjected to some environmental mutagen.

Nothing is planned. It is all spontaneous.

ETA:  Causes of mutations