The deification or worship of anyone or anything other than Allah, or more literally the establishment of "partners" placed beside Allah.
Its just partners with Allah when you know that Allah is one.
E.g Hazrat Abu Bakr (RA) was an idol worshiper because at that time, he didn't knew about Allah being one so he was not a sinner but then Muhammad (PBUH) came with the message of Allah being one so Hazrat Abu Bakr (RA) admitted his error and submitted to Allah. Now if he commit shirk then its an actual sin but its not an unforgivable sin . Allah says that I can forgive any sin but if you have committed shirk while you know that Allah is one and didn't repent before dying then yes, Hell.