margin question for newbie

hello all,
great site and hope to learn all about forex, i have a basic question, i am a bit confused about the margin and free margin on the smartlive mt4 platform, i am new to trading and have a demo account, could anybody explain how it works,
thanks in advance,
learner1

In MT 4 you can see this in a terminal window:

Balance - money you have before opening a trade.
Equity - money what you will have if you close your trade now.
Margin or blocked margin - money blocked for your trade as a deposit.
Free margin - money you can use to open a new trade.