I read in a book "It is permissible for one to ask that one's duas[supplications, prayers] be accepted for the sake of the Awliyâ[Dear slaves of Allahu ta’âlâ]." What is meant by this, i havent heard it before?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

We did not exactly understand what you mean. If you are asking about a person requesting another living person whom he thinks is righteous to supplicate for him, then this is permissible as evidenced by many Islamic texts, including the Companions seeking the intercession of al-Abbaas to supplicate for them for rain, and also seeking the supplication of Yazeed ibn al-Aswad.

Moreover, the Prophet ordered whomever meets Uways to ask him to supplicate Allah for him [that person] for forgiveness, and other pieces of evidence.

‘Umar ibn al-Khattaab narrated: “I sought permission of the Prophet to perform ‘Umrah. He gave me permission and said: “My younger brother, do not forget me in your supplication.” He (‘Umar) said: He told me a word that pleased me so much so that I would not have been pleased if I were given the whole world. The narrator Shu'bah said: “I then met Assim in Madeenah. He narrated to me this narration and reported the wordings: “My younger brother, share me in your supplication.” [Ahmad, Abu Daawood and at-Tirmithi]

Shaykh Ibn Taymiyyah said:

“Whoever said to another, 'Supplicate Allah for me or for us,' and his intention is to benefit that person and to benefit himself as well, and that person does what he was commanded to do – just as he (may) command him to do any other good deed – then he (the one asking another to supplicate for him) is following the example of the Prophet, sallallaahu ‘alayhi wa sallam. However, if his intention is only to ask for his need without intending to benefit the one he commands (by making him earn reward for performing worship through supplication), then he is not following the example of the Prophet, sallallaahu ‘alayhi wa sallam. Rather, this is considered a less preferred request. It is more preferred, in this case, to aspire for the rewards of Allah, the Exalted, and (following his) Messenger than to aspire to people and to ask them. All this falls under the category of the permissible and prescribed request for the living people to supplicate Allah in one's favor.” [End of quote]

If what you mean is to ask for supplication from the dead person who is believed to be one of the Awliya of Allah, and asking him to supplicate Allah for him, believing that his prayer is answered, even if he is in his grave, then this is a false belief and a strong reason for Majar Shirk.

Shaykh al-Islam Ibn Taymiyyah said: “The same applies to the Prophets and the righteous people, even if they are still alive in their graves and able to supplicate Allah in favor of the living, and even if reports were narrated to this effect, it is impermissible to ask the dead to supplicate on behalf of the living. None of the righteous predecessors had done so because it leads to Shirk by associating the dead with Allah in worship or worshiping them instead of Allah; contrarily to asking anyone of them while they were alive as this does not lead to Shirk.” [End of quote]

Allah knows best.