Aag ka dariya is a blend of two different words which came from distinct languages. Aag is a hindi word whose meaning is fire while dariya is a persian word used in urdu also which means sea. Together it is referred as the Sea of fire. Sometimes large rivers are also called dariya due to their vast extent.
One of the famous hindi quote is:
Ishq Nahi Aasan Itna Samajh Lijiye
Aag Ka Dariya Hai Aur Doob Ke Jaana Hai
whose meaning is as follows:
Love is not easy consider it as
the sea of fire and we have to cross it submerged.