Forex support is a virtual line, a line that in reality does not exist, that "creates" the support level for a currency over the day, over the week, month and year.
The support line is the line that most traders use to know where the price is likely to hit in the near future so they can trade accordingly to that line.
Each Forex website and trader creates their own support line to have an expectation where the price will hit.
FX Street for example, has a daily analysis of where the price is likely to hit at the bottom so traders can take advantage of this free service and trade at their own risk considering their support line.
What is the support line good for? Traders who want almost sure profits with minimal risk, can use the support line to place a long position if the price hits the support and then close the trade when the price hits the resistance line.
Support lines are very useful because they usually consider a lot of indicators and Fibonacci reversal points to indicate the traders where they should long that currency pair.
There is another site, FX Pros, which have their own support lines. You should try as many sites as you can with a demo account and see where their lines are the most accurate and then use your real account to place the trades.