Here is the translation of the meaning of this great soora:
[When the revelation was brought down to the Prophet – salam to him – at the start, he conceived doubt about it when he saw his people denying and ridiculing him, and they said to him: “The one who brings the revelation to you is one of the genie, and is not an angel.”
So God wanted to confirm his heart for his mission and to remove the doubt from him in order to carry out the mission with zeal, and with a firm will and determination; therefore, He revealed to him many revelations to confirm him to carry out the mission; among such revelations were these abbreviated letters:]
1. Ya. Sien.
[These abbreviated Arabic letters mean:]
(Be Certain and Comply.)
So (Be Certain and Comply)
2. [I swear] by the Quran and its wisdom!
3. Surely, you are one of the messengers.
4. [Following] on a standard [straight and correct] way.
5. A revelation [coming] from [God] the All-Mighty, the Most Merciful.)