Uddhacca is translated as 'agitation' or 'restlessness' and kukkucca as 'worry' or 'flurry'. Uddhacca arises with each and every type of akusala citta. It prevents the citta from wholesomeness.
As regards kukkucca, worry, the 'Visuddhimagga' (XIV, 174) states:
'...It has subsequent regret as its characteristic. Its function is to sorrow
about what has and what has not been done, It is manifested as remorse.
Its proximate cause is what has and what has not been done. It should be
regarded as slavery.'
When we have done something wrong or we have not done the good deed
we should have done, we might be inclined to think about it again and again.
We may ask ourselves why we acted in the way we did, but we cannot
change what is past already. While we worry we have akusala cittas; worry
makes us enslaved. Uddhacca and kukkucca prevent us from being tranquil.
As regards vicikicchā, doubt, there are many kinds of doubt. One may have
doubts about the Buddha, the Dhamma and the Sangha, or doubt about the
Eightfold Path. Doubt is akusala and a hindrance to the performing of Kusala.
All of the hindrances are obstructions to the performing of kusala. Is there a
way to eliminate them? Samatha or the development of calm is a way to
temporarily eliminate the hindrances. The calm which is developed in samatha
has to be wholesome calm, it cannot arise with akusala citta. There is a
degree of calm with each kusala citta but it is hard to know the characteristic
of calm precisely, because there are bound to be akusala cittas very shortly
after the kusala cittas. In order to develop the calm which is temporary
freedom from the hindrances, right understanding (paññā) is indispensable. If
one merely tries to concentrate on a meditation subject without right
understanding of kusala and akusala and of the characteristic of calm, calm
cannot grow. The paññā of samatha does not eradicate defilements, but it
knows the characteristic of calm and it knows how it can be developed by
means of a suitable meditation subject. Akusala citta is likely to arise time and
again, also when one applies oneself to samatha. One may be attached to
silence and then there is akusala citta instead of the calm of samatha. Or one
may think that when there is no pleasant feeling nor unpleasant feeling but
indifferent feeling there is calm. However, indifferent feeling can arise with
kusala citta as well as with akusala citta; lobha-mūla-citta can be
accompanied by indifferent feeling and moha-mūla-citta is invariably
accompanied by indifferent feeling. Thus, when there is indifferent feeling it
may seem that one is calm, but there is not necessarily the wholesome calm
of samatha. The paññā of samatha must be very keen in order to recognize
the defilements which arise, even when they are more subtle.