What is concern? From taking its stand on non-attachment, non-hatred, and non-deludedness, coupled with diligence, it considers whatever is positive and protects the mind against things which cannot satisfy. Its function is to make complete and to realize all worldly and transworldly excellences.
It is intelligence which realizes the positive and protects the mind from what is unreliable by persevering in diligence and not falling prey to the emotions.

According to the Bodhisattvabhumi, there are five kinds:

1.     Concern with regard to things in the past
2.     Concern with regard to things in the future
3.     Concern with regard to things in the present
4.     Concern with regard to things which were to be done before
5.     Concern with regard to things which continue together with what is done now
They have been explained by Tsong-kha-pa as follows:
To get rid of the evil done in the past by means of the teaching,
And in the future to think of doing what is positive, and, in the same way,
In the present to do acts without being absent-minded,
And to carry oneself in such a way that evil will not rise,
And to gain control over oneself in thinking that all this is possible,
Then, in view of the above facets, one acts appropriately.
Since it has been said that its function is to provide a basis for letting the worldly and transworldly excellences all be present, it is very important as the basis of all the levels and paths. In the same way, Nagarjuna (Suhrllekha 13) states,
The Buddha has stated, "Concern is the basis of immortality,
Negligence is the state of death."  Therefore,
In order for you to increase the wholesome,
Always be concerned and be so devotedly.