Margin defined as more purchasing power, than you have in your account, given to you by your broker. The terms 'Margin' and 'Exposure' are used interchangeably. If a broker is providing '2x exposure' that is the same as saying that the broker is providing '50% margin'.
To understand this better, here's an example:
Assume that you have ₹10,000 in your Trading account, and your broker is providing you 50% margin (2x exposure) on Futures.
This means that if you want to place a trade worth ₹10,000, and the Exchange requires the total amount of ₹10,000 from the broker, you would only be debited for 50% of the trade value (₹5,000) on the transaction. However, by the end of the trading day, you must exit your position. Many brokers require a 'minimum' or 'initial' margin; this is the minimum amount that you must maintain in your Trading account at all times.
For example, a broker might require ₹10,000 as an initial margin; this means that you must have at least ₹10,000 in your Trading account at all times. Brokers provide different levels of exposure (different margins) in different segments of products. So while a broker may provide 4x exposure on Equities, they may provide 2x exposure on Futures and no exposure on Options.