'Reflection' in Kierkegaard is opposed to 'immediacy'. It's the state in which you form an idea about something and deal with the idea, as opposed to dealing with the thing itself.
For instance, The Present Age starts with:
The present age is one of understanding, of reflection, devoid of passion, an age which flies into enthusiasm for a moment only to decline back into indolence.
Not even a suicide does away with himself out of desperation, he considers the act so long and so deliberately, that he kills himself with thinking -- one could barely call it suicide since it is thinking which takes his life. He does not kill himself with deliberation but rather kills himself because of deliberation. Therefore, one can not really prosecute this generation, for its art, its understanding, its virtuosity and good sense lies in reaching a judgment or a decision, not in taking action.
Reflection in Kierkegaard is related to 'abstraction' and 'objectivity'. 'Abstraction' is when something exists as an idea instead of as a real thing, and 'objectivity' is when a person thinks they can deal with something as though it was an idea and not a real thing.