As-Salaam alaikum,
On the authority of Tamim al-Dari, may Allah be pleased with him, it was reported that the Prophet, Sallallahu alaihi Wasallam, said:--

"Religion is Nasihah." We asked, "To whom?" The Prophet, Sallallahu alaihi Wasallam, said:
"To Allah and His Book, and His Messenger, and to the Leaders of the Muslims and their
Common folk." (Sahih Muslim).

Some look at the word in this Hadith, 'NASIHAH' as referring to sincerity... but Nasihah is indeed a broad concept in Arabic Language. Besides sincerity, Nasihah also means to truly seek the best in terms of intention and action, for the one whom one is making Nasihah to. In this brief Hadith, the Prophet, alaihi-s-Salaat was-Salaam, made a profound statement describing the essence of Islam. Nasihah, therefore, forms the bulk and the most important pillar of Islam since it encompasses Islam, Imaan and Ihsan.

Nasihah to Allah:
Here, we should fulfill Allah's Obligations on us in the best way possible (Ihsan), and this refers to/includes:--
(a) Believing in Allah and denying any partners with Him.
(b) Believing in His Attributes.
(c) Obeying Him and
(d) Fulfilling His Commands and Abstaining from what He forbids.

Nasihah to His Book:
This should include--
(a) Believing that the Holy Qur'an is from Allah Ta'ala, that it is His Word and it is not like the word of man.
(b) To read/recite the Qur'an according to one's ability, study its admonitions, lessons and parables.
(c) Calling others to believe in the Qur'an, defend and protect it from any kind of distortion and misinterpretation.

Nasihah to His Messenger: This includes:--
(a) Believing the Prophet's Message, Sallallahu alaihi Wasallam.
(b) Believing in all that he brought as being Divinely Inspired.
(c) Loving him more than we love ourselves and our families-- it is the second level of love after the love of Allah.
(d) To accept him as the true leader and only human's final word with respect to the religion.. he is the only true human authority and everyone else's statement or opinions come after his.

Nasihah to the Muslim Leaders:
Leader here stands for both 'Ulama and those in authority at all levels. And Nasihah to such leaders shuld include:
(a) Helping them in whatever is good or beneficial.
(b) Obeying them in what is right; and reminding them if they err or forget.
(c) Being patient with them if they do things which we dislike- we try to do nasihah and at the same time, we tolerate the Leader, as otherwise it amy lead to instability in the Muslim Community.

Nasihah to the Common Folk:
This includes observing the rights of other Muslims and fulfilling our obligations towards them. These obligations differ depending on the group of Muslims (eg our parents, children, relatives, neighbors etc)- greeting them, visiting them when they are sick, making Du'a for them, giving advice if they ask for it...
We should also sacrifice our time, effort, or money if need be, for the betterment of the Muslim Community.

When you explore all the obligations mentioned above, we see that Nasihah encompasses everything in Islam, Imaan and Ihsan.