Knowledge of its meaning in terms of negation and affirmation. Also knowledge about whom we are worshipping. The evidence for this is the saying of Allah (عز و جل ) in the Holy Qur'an;

“So know, that ‘La ilaha illallah’ (there is no ilah worthy of worship except Allah)”. [Muhammad 47:19]

In a confirmed Hadith, narrated by Uthman (رضي الله عنه ) the Messenger of Allah (صلى الله عليه و سلم ) said:
“He who dies knowing that there is no deity worthy of worship except Allah shall enter Al-Jannah (Paradise)”. [Sahih Muslim]

Allah (عز و جل ) says:

“And We did not send a Messenger before you (O Muhammad) but We revealed to him (saying): La ilaha illallah Ana (there is no ilah worthy of worship except I (Allah), so worship Me”. [Al-Anbiya 21:25]

Prophets like Nuh, Hud and Salih (as) each said:

“…O my people! Worship Allah! You have no other ilah but Him”. [Al-A’raf 7:59, Hud 11:50, Hud 11:61]

And The Exalted also tells us that the da’wah of all the Prophets and Messengers was one and the same:

“And verily, We have sent among every Ummah (nation) a Messenger (Proclaiming): Worship Allah (Alone), and keep away from Taghoot (false deities).” [Al-Nahl 16:36]
 
And He also tells us about the friend (Al-Khalil) - peace and blessings be upon him:

“And (remember) when Ibrahim said to his father and his people: “Verily, I am innocent of what you worship, Except Him (Allah) Who did create me; and verily, He will guide me.” And he made it (La ilaha illallah) a Word lasting among his offspring, that they may turn back (i.e. in repentance to Allah).” [Az-Zukhruf 43:26-28]

Allah the Exalted, ordained His Prophet Muhammad (صلى الله عليه و سلم ) to convey to his Ummah:

“…And your Lord has decreed that you worship none but Him….” [Al-Isra 17:23]

And:

“Worship Allah and join none with Him in worship…” [An-Nisa 4:36]

These above ayat and others in the Quran, similar in meaning, give a clear indication that all the Prophets and Messengers were sent to their nations and entire mankind to prove ‘La iIlaha illallah’ - there is no deity (worthy of worship) except Allah.