Fiqh Muamalat is the part of Fiqh dealing with relations between people, such as the Islamic commercial law. It is the area of fiqh which governs the review process for Islamic financial products from the perspective of Shari'ah-compliance and does not have legal force in countries without explicitly Islamic legal systems that cover commercial transactions.
Muamalat is set of rules (fiqh) related to worldly matters such as business/trading/commerce transactions, lending and borrowing contracts.
Muamalat nowadays, is always associated to economics and finance since these elements are the key for happiness in this world and hereafter.