People don't agree on the definition of knowledge, yet most people still require that a person have knowledge of a subject before becoming a professor in it or practising in the profession (like a doctor).
So how is this significant to Islam? Well it shows that not everyone who to agree upon a thing, for it to be applied in Islam, or for it to be a necessary source of law.
I have come across people who are ignorant about much of Islam, and love to follow their own desires and make their own new 'Islam' and follow the modernist sect. When they are told that what they are doing is against the ijma (the consensus of the scholars on a particular topic, and ijma is one of the 4 sources of Islamic law), they become angry and retort/reply that people have not agreed upon the definition of ijma, and that there are different definitions about how many people are required to form a consensus. Thus these modernists say that ijma can't be a source of Islamic law.
However they can't use this weak and fallacious argument against ijma, as I showed above. They believe in many things (like knowledge) that are not agreed upon by everyone, yet it is required for people to know what is right, and give an Islamic opinion. They even believe in Allah, even though many people reject Allah. And they believe in secular law and democracy and culture (they even prefer culture to Islamic sources of the Qur’an and Sunnah in making law), even though there is no agreement or single definition on any of them. And even they believe in reason, even though there is no single definition or agrement on it.
Furthermore, why would the Prophet talk about ijma, when its not a source of law, and doesn't mean anything?
There are many Hadiths of the Prophet on the ijma and show the importance of following the group that is the majority, here are some:
"Allah's hand is over the group, follow the largest group, for verily whoever dissents from them departs to hell" Tirmidhi from Ibn `Abbas
"You have to follow the congregation for verily Allah will not make the largest group of Muhammad's community agree on error" (Ibn Abi Shayba)
"Shaytan is a wolf like the wolf that preys on sheep, taking the isolated and the stray among them; therefore, avoid factionalism and keep to the congregation and the collective and the masjid." (Ahmad)
"Verily Allah will not make my community -- or Muhammad's (saw) community -- agree on error, and Allah's hand is with the largest congregation." Tirmidhi said: "And the meaning of "jama`a"/community according to the people of knowledge is: the people of jurisprudence, learning, and hadith
So you see that the reasoning of this modernist sect is illogical, self-contradictory and opposes Allah swt and the Prophet (Sallallahu alaihi wa sallam). No wonder it has arisen at a time when Islamic knowledge has fallen, and arrogance has increased amongst people.