Forex trading is the largest market in the world, with nearly $2 trillion traded on a daily basis. There are many factors that can contribute to changes in the value of a currency. Some of these factors include terms of trade, sometimes referred to as the balance of trade, which is when there's an improvement in the terms of the trade thanks to the price of a country's exports being higher than the prices of its imports. Other facts include differences in inflation rates, which basically involve the value of the currency, and public debt, which typically occurs when foreign investors lose confidence in the economy and make fewer or no investments and leads to inflation and devaluation of the home country's currency.
Brokers use margin levels in an attempt to detect whether FX traders can take any new positions or not. Different brokers have varying limits for the margin level, but most will set this limit at 100%. This limit is called a margin call level. Technically, a 100% margin call level means that when your account margin level reaches 100%, you can still close your positions, but you cannot take any new positions.
So, for an investor who wants to trade $100,000, a 1% margin would mean that $1,000 needs to be deposited into the account. The remaining 99% is provided by the broker. No interest is paid directly on this borrowed amount, but if the investor does not close their position before the delivery date, it will have to be rolled over. In that case, interest may be charged depending on the investor's position (long or short) and the short-term interest rates of the underlying currencies.