Every cultivator needs to have the Three Non-Outflow Studies of precepts, samadhi, and wisdom.
Precepts: Although there are the Five Precepts, the Ten Precepts, the Bodhisattva Precepts, and so on, the Five Precepts are the most fundamental. They are: do not kill, do not steal, do not engage in sexual misconduct, do not engage in false speech, and do not take intoxicants. If one can hold the precepts, one will not make transgressions.

What is samadhi? Ordinarily, samadhi refers to Dhyana (Chan) samadhi. Generally speaking, it means "unchanging." When some people cultivate, they do a lot of idle thinking. Today they practice Chan meditation, but tomorrow they hear that reciting the Buddha's name has a lot of merit and virtue, so they give up Chan and go recite the Buddha's name. Two days later they hear the merit and virtue of reciting mantras is supreme, so they stop reciting the Buddha's name and start reciting mantras instead. In general, that's how they carry on. Cultivating this Dharma-door today and that Dharma-door tomorrow, they end up achieving nothing. There are also some people who recite the Buddha's or Bodhisattva's name with the attitude of "sunning it one day and freezing it for ten." They recite today, but quit tomorrow. This is because they have no samadhi. Samadhi is extremely important in our cultivation. If we have no samadhi, we will surely have no success in cultivation. Moreover, if we do not have sufficient samadhi and we lack a firm resolve in the Way, external temptations can easily influence us and cause us to fall.

Next there is "wisdom." In the world, there are wise people and stupid people. Why is it that some people are smart and some are stupid? Of course, it involves cause and effect. All those who cultivated by reciting the Buddha's name in former lives have comparatively more wisdom in this life. But those who didn't plant good roots in the past have less wisdom than most people.

The Great Learning says,

When a person has concentration, he can be still.
When he is still, he can be at peace.
When he is at peace, he can reflect.
Upon reflection, he can obtain what he wants.

If you are in samadhi, you can produce wisdom. But if you are not concentrated, if you produce droves of scattered thoughts, if you are hasty and flighty, then how can you calmly distinguish right from wrong and understand the truth?

Precepts are rules which keep us from committing offenses. While refraining from committing offenses, we should also cultivate the Way, and the secret to cultivating the Way is samadhi power. Once you have samadhi power, you can develop wisdom, and if you can progress one step further, you can understand the Way and become a Buddha. That is why the three studies of precepts, samadhi, and wisdom are the essential requirements which all people who cultivate the Way should possess. If you do not hold the precepts, then you can create offenses and call forth karma. Lacking samadhi power, you will not be able to accomplish cultivation of the Way. You will not have any wisdom and will become dull-witted.
I exhort you all to take special note of the three studies of precepts, samadhi, and wisdom in your cultivation; always use them to alert yourself and I believe it will help you in your cultivation.