There are three kinds of Tawheed (oneness of Allah), that must be implemented.
The first is: Tawheed al uluhiyya. That means that Allah s.w.t is the only one, that has the right to be worshipped. He’s the only true God, all other are false goods. No act of worship can be done to anyone exept Him s.w.t, like prayer, slautgher or anything else of worship. This means also, that the worship that’s been done, must be done at the way He tabaraka wa ta’ala wants it to be done. For that we need, a evidence from the Qur’an, Sunnah or the consensus of the sahaba.
The second is: Tawheed al rububiyya. This means that Allah s.w.t is the only Creator, Sustainer, Lord. That’s something that the polytheists believed in.
And indeed if you ask them, “Who has created the heavens and the earth?” They will surely say: “The All-Mighty, the All-Knower created them.” surah 43 verse 9.
And if you (O Muhammad صلى الله عليه وسلم) ask them: “Who has created the heavens and the earth,” they will certainly say: “Allah.” Say: “All the praises and thanks be to Allah!” But most of them know not. surah 31 vers 25
And if you were to ask them: “Who has created the heavens and the earth and subjected the sun and the moon?” they will surely reply: “Allah.” How then are they deviating (as polytheists and disbelievers)? surah 29 verse 61
And verily, if you ask them: “Who created the heavens and the earth?” Surely they will say: “Allah (has created them).” Say: “Tell me then, the things that you invoke besides Allah – if Allah intended some harm for me, could they remove His harm? Or if He (Allah) intended some mercy for me, could they withhold His Mercy?” Say: “Sufficient for me is Allah; in Him those who trust (i.e. believers) must put their trust. “ surah 39 verse 38.
The third is: Tawheed al asma’ wa assifaat. This means that Allah s.w.t is the only that has these names and attributes. A person can have a name, like example Hakeem (the wise) and Allahs name is Al Hakeem. The person can also have that attribute, but he isn’t wise the same way like Allah, because Allah s.w.t is supreme in everything.
There is nothing like Him; and He is the All-Hearer, the All-Seer. surah 42 verse 11
Do you know any one equal to Him? (That has the right to have the same name) surah 19 verse 65.
If any of this three is missing, the tawheed hasn’t been established completely, and a person isn’t a real muwahid