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. The amount of gold players currently hold is indicated in the top left of the skill tree.
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.