Respec is a term for respecialisation and refers to a game feature that allows players to partially or completely reallocate the points they have spent in various skills, 'talents', or 'attributes'.
Players in Diablo 4 will be able to respec their characters at any point on the game and reset skills on the skill trees. This is achieved by right-clicking on the skill as show below:
Skills will be able to be reset on a point by point bases or via "Refund All" button at the bottom of the skill trees.
The only requirement to respec skills is a cost in gold.
As players level, the cost of respecing becomes more expensive. At a certain point, the cost will become very expensive and player may decide that it's better to actually re-roll their character from scratch.
Costs of respecing skills is not known at this point.