A stretch goal is an additional goal in case the initial goal has been exceeded – normally in a fundraising context.
For example, an initial goal could be “raise $10 for sunscreen research in two days”.
And if the target was reached on the first day, the fundraiser could introduce a ‘stretch goal’, for example, ‘raise an additional $5, so $15 in two days’.
Goal setting in any context involves predicting what might be possible – perhaps a target that is ambitious but realistic – and then taking steps towards it.
A goal reached ahead of time could reflect an underestimate in the initial planning, or a target that was not ambitious enough (or others – what are other possibilities?)
Whatever the case, the next step should be to reorient towards a higher target (a stretch goal).