Metatrader volume is the amount that you will trade on the symbol you are trading. So basically, it is how much you are putting on the trade.
Volume, Lots, Risk
Metatrader volume is also simply known as Volume, but also interchangable for Lots and can also be termed risk (as increasing volume inherently increases risk per point/pip if the stop loss isn’t moved).
Is MetaTrader Volume the same in MT4 and MT5?
The concept is the same, however a broker may use different measurements between symbols, which you can check via the properties area. Depending on what you are trading, 1 standard lot can be equal to 100,000 forex units, or 10,000 index units. So trading 1.4 lots of Forex means trading the value of that symbol to the value of 140,000.
Can I change the volume that is set when I place orders?
You can change the volume manually, however you can also set a default value if you know the value (lots) you wish to use when you are trading. Check here to see how to change the default lots for your orders.