Prapanna is a sanskrit word and it means complete surrender/dedication. To make it easier for all kinds of people, Raamaanuja of VishishTaadvaita said that a person has to be just a 'prapanna', meaning completely surrendered to God. This is called 'Prapatti'. If a person completely trusts God and surrenders himself in thought, word and deed, then he can attain to the highest point "Vishnoh Paramam Padam".
There is mention of Prapanna many times in Bhagavata Purana. A prapanna is expected to fulfill some or all of the five a~ngas to attain prapatti. 'Prapanna' is the one who surrenders herself/himself to God and how she/he deals with life treading a path of purity.
It also refers to a person
References
Prapanna Wikipedia(Text) CC BY-SA