What exactly is this Tawheed that is so important, that which holds such a high position?

Linguistically, the word Tawheed comes from the word wah-hada, which means to make something waahid (singular). Waahid is the opposite of two, three, etc., the opposite of plurality. So waahid is something that will continue to be singular and never become a partner of something else.

As for the Sharee’ah’s definition, then Tawheed means to single out Allaah with all forms of worship, so that you make all of your worship for Allaah alone. Allaah says :

“and the religion will be entirely for Allaah"[1]

The word religion in this verse means worship, as Allaah says:

“and I only created the Jinn and Mankind to worship Me"[2]

and Allaah says :

“and worship Allaah, and do not ascribe a single partner to Him"[3]

Allaah also says:

“So call on Allaah, making the religion entirely for Him, even if the disbelievers detest it” [4]

This is the meaning of Tawheed - to single out Allaah in all acts of worship and to abandon the worship of anything else.

FOOTNOTES:
[1] The meaning of Soorah Al-Anfaal (8):39
[2] The meaning of Soorah Ath-Thaariyaat (51):56
[3] The meaning of Soorah An-Nisaa’ (4):36
[4] The meaning of Soorah Ghaafir (40):14