Why we use Radhiallah Anhu or Anha with the name of companions of Prophet Muhammad SAW?

Radhiallah Anhu or Radhiallah Anha means May Allah be pleased with him/her. It is added after the names of the sahabah (companions of Prophet Muhammad ﷺ) and wives of Prophet Muhammadﷺ as a sign of respect. It is often represented by the acronyms(RA or R). 

Raziallahu Anhu is used for male companions and Raziallahu Anha is used for female companions of Prophet Muhammadﷺ.

Arabic text - 

Radhiallah Anhum رضي الله عنهم (for plural male) and

Rathiallah Anhu رضي الله عنه (for singular male)

Rathiallah Anha رضي الله عنها (for singular female)