'Instrumental goals' or 'instrumental values' are things that an agent wants for the sake of achieving other things. For example, we might want to get into a car, not because we enjoy the act of opening car doors for their own sake, but because we want to drive somewhere else.

'Terminal' goals, values, or preferences are those where the preference is derived locally rather than by looking at further or distant consequences. If you enjoy eating chocolate (and otherwise approve of this enjoyment, etcetera) then you aren't deriving your preference based on what you believe to be the further consequences of eating chocolate.

Imagine reality as an enormous web of events, linked by cause and effect. "Terminal value" is usually local and be evaluated at a single event inside the graph; even if it's a nonlocal good thing, we'd evaluate it by evaluating the history up to some point, and then we'd have a chunk of definite goodness that would stand on its own no matter what happened later.

"Instrumental value" is a nonlocal property of an event, depending on its real or expected future, and contingent on that future; if you add up all the instrumental values on the graph, you don't get a meaningful sum because you may be double-counting some value.

On a moral or ethical level, instrumental values are justified by appealing to their consequences, while terminal values are justified without appeal to their consequences.

Further reading:

Terminal versus instrumental goals / values / preferences