Mt4, or MegaTrader 4 is a comprehensive trading platform that can be used for trading in the futures, CFD and FOREX markets. The platform was designed for banks, brokerages and financial institutions, but can in fact be used by anyone who is interested in trading in these markets. An Mt4 indicator provides charts, graphs, and other vital information that indicate what a trader should do, i.e. buy, sell, buy long, buy short, etc. Indicators can save traders much time and effort by doing a lot of the research and 'groundwork' for them, so all they have to do is look at the indicator and, based on the information, enter or exit their trade. Simply put, indicators help traders become more successful and more profitable. There are currently over 1000 indicators for the Mt4 platform, and each one gives you slightly different information. For example, the Advanced ADX indicator tells you when to buy and when to sell. The histogram that is displayed is color coded red and green. Red means sell, green means buy. So when you look at the current prices that are displayed directly above the histogram, you'll know whether you should enter or exit your trade. An example of the Advanced ADX indictor is below: 
Another example of an Mt4 indicator is the color_Stochastic.mq4. This indicator shows oversold and overbought levels. This can help traders spot trends and gives a prediction of an uptrend or downtrend. An example of the color_Stochastic.mq4 is below: 
All indicators have 'rules' or objectives to follow. It's important to choose an Mt4 indicator that fits with your trading style. The indicators are free to download and can be found easily on the internet. While indicators can give excellent buy and sell signals, many people use a combination of indicators to come up with their own trading strategies. This generally comes after they've been trading for some time, and have gained some knowledge of how the markets work. For beginners, there are indicators that are easy to understand and come with complete instructions on how to use them most effectively. As you gain more experience and knowledge, you'll want to use indicators that contain more technical or complex trends and data. In order to trade using the Mt4 platform and Mt4 indicators, you'll need to open a trading account with a brokerage or firm that has an Mt4 platform. This information is easily found on the internet. Many of these brokerages offer an easy registration process and a low minimum account size, sometimes as low as $1. One of the biggest advantages of Mt4 indicators is that it allows you to automate your trades and your positions. Most people don't have time to constantly sit in front of their computers and watch the markets. You can 'set it and forget it,' meaning you can set up your trade, as well as your enter points and exit points to automatically occur based on parameters that you set. As you get more comfortable with the trading process, you will learn to use your indicators as guidelines, and not rely on them to give you literal information. Remember that in the world of trading, anything can happen and your USD/CAD pair could lose pips instead of gain them. On the flip side, indicators give a much more accurate picture of what is happening and much more dependable data than other software programs. No matter what your FOREX trading style of charting: candlesticks, Fibonacci, or other fundamental method, you'll be able to find an indicator that will fit your style. You may have to experiment with several indicators at first in order to find one that you are completely comfortable with. Many traders use more than one indicator for their overall strategy. It's important to keep in mind that indicators are tools only, albeit very useful tools. They can only give you information and advice based on trends and past history; they are not a guarantee of success. If you are going to become a trader, you will need to educate yourself on the market and how to decipher technical data and charts. There are some very good online courses and books on the subject of FOREX trading.