Thi asked:

What does it mean to have "existential doubts"?

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Existentialists do not have 'doubts'. They suffer from psychologically more interesting conditions like
anxiety, vertigo, nausea.

Unlike these other, philosophically more subversive concepts, existential doubt does not disclose
anything about the nature of ultimate reality, or the ground of our being, or lack of it. Existential doubt
is an ubiquitous, cliché-ridden theme of popular culture. Like the bored executive on the
thirty-third floor who wakes up one day to the realization that he 'could have been more'. Or the
teenager sitting up all night staring at their fish tank. Or...well, I won't bore you.

It is a tragedy when someone who's life is perfectly all right,who has no reason to succumb to
existential doubts, falls victim to the gnawing worry, 'Why am I doing this? What's the point of it all?'
When a person whom we think really ought to feel doubts about their life doesn't, it is a comedy.

Existential doubt is just one particular example of the dialectic of scepticism, doubt and certainty. The
point has often been made in discussions of the problem of philosophical scepticism that progress in
human knowledge depends upon our willingness to doubt and question, to explore alternatives, to
demand justifications. There are also times where doubt is merely pathological. The same pair of
alternatives applies to the case of so-called existential doubt.

Geoffrey Klempner