Muraqabah is when a person watches over or takes care of his spiritual heart (or soul) knowing that Allah subhanahu wa ta'ala is watching over us.
Allah subhanahu wa ta'ala says,
"...Be assured that Allah knows what is in your hearts. So, fear Him and be assured that Allah is most Forgiving, Forbearing." (Al-Baqarah(2) : 235)
"..And Allah is watchful of every thing" [Al-Ahzab(33) : 52]
"...He is with you wherever you are, and Allah is watchful of whatever you do. (Al-Hadid(57) : 4)
In the hadith of Jibreel alayhis salam when he asked the Beloved Prophet Salla Allahu ta'ala 'alayhi wa Sallam about ihsan (goodness and excellence), the Beloved Prophet Salla Allahu ta'ala 'alayhi wa Sallam replied,
"Ihsan is to worship Allah as if you see Him, but since we do not see Him we should know that He sees us at all times."
[Sahih al-Bukhari and Muslim]
The meaning of this hadith is the definition of muraqabah. Namely, the endurance of the servant's knowledge and his conviction and certainty that Allah is watching over his internal and external affairs. To have this knowledge and certainty at all times is called muraqabah. It is the fruit of the servant's knowledge that Allah is his Watcher, Over-seeing him, Hearing his utterances, and Observing all of his deeds at all times.
"Drown deep into yourself and find the secret of life"
The Messenger of Allah Salla Allahu ta'ala 'alayhi wa 'aalihi wa Sallam said:
"Whoever possesses the following three qualities attains the sweetness of faith: To have Allah and His Messenger dearer to him than anything else, to love a person only for the sake of Allah, and to hate to return to kufr after Allah has rescued him from it like he hates to be thrown into fire."
[Sahih al-Bukhari and Muslim]
"Those who believe, and whose hearts find peace in the remembrance of Allah: for without doubt in the remembrance of Allah do hearts find peace. (Al-Rad(13) : 28)