Hi @asmeedhungana,
I suggest reading the “What is Reentrancy” section (and also the whole post) from Reentrancy After Istanbul.
Sending Ether from Contract A to Contract B involves Contract A calling functionality on Contract B which depending on the available gas and mechanism used, could result in Contract B calling back into Contract A.
This is only for a contract calling an address that is also a contract, and not for a contract calling externally owned accounts.