Praise be to Allah, the Lord of the Worlds, and peace and blessings be upon our Master Muhammad, the Faithful and the Honest.
The precise fact is that "Ar Rida" is to be pleased with Allah. However, this definition raises a question: Given I seek Allah's Pleasure, how can I be pleased with Allah?
The answer is: You will be pleased with Allah when your pleasure in receiving hardship is equal to your pleasure in receiving graces. Everybody is pleased with Allah when things are going well, like health, money, wealth, wife, great house, etc. However, people are sifted according to the way they deal with calamities. Accordingly, heroism lies in being pleased with Allah in prosperity and adversity, in bestowal and prevention, in health and illness and in affluence and poverty; indeed this is heroism and this is the deep true knowledge. When man is pleased upon being tried with hardship, it is a medal of honor on his chest.
The normal reaction of man to pain, poverty, oppression, illness and adversities is to be anguished, but when man knows beyond doubt that Allah shall never give up on him, he will be pleased. Be aware that Allah will never give up on you; otherwise, He does not deserve to be worshiped. Allah says:
﴾ ﴾ And to Him return all affairs (for decision)﴿ ﴿
To Allah return all affairs on earth in the five continents since the birth of Adam till the Day of Ressurrection:
﴾ ﴾ And to Him return all affairs (for decision). So worship Him (O Muhammad PBUH) ﴿ ﴿
When does Allah ask you to worship Him? He does so after He assures you that all affairs are in His Hand, including your life, death, health, illness, wealth, marriage, happiness with your family and everything else.
We own the free will, thus when you believe, as aforementioned, that everything is in Allah's Hand, you are practicing Tawheed, needless to say that the one who believes in Allah alone is in inner peace. The Prophet PBUH said in the Hadith narrated by Abdullah bin Mas'ud:
(( (Whoever focuses all his concerns on one issue, the concerns of the Hereafter, Allah will suffice him and spare him the worries of this world…) ))
Do everything for the Sake of Allah, and Allah will relieve you of all other concerns.