Position represents an expression of a market commitment that a trader has. For example, if you own 2 BTC, you have a long position of 2 BTC.
Position is opened when an order to open this position is executed. In other words, you first place an order. And when that order is filled, you have a position. You can close it when an order of the opposite direction, essentially canceling this position, is executed. At that point, your unrealized gains or losses turn into realized.
Positions can be Long and Short. The former bets that the price of an underlying asset will go up and represents Buy position. The latter bets that the price of the underlying asset will go down and represents Sell position.
Note: Here you can check a more detailed article on how to benefit when the price moves up and when the price moves down.
The size of a position is measured by the value of an underlying asset. Of course, when you own some asset outright, as in our example above, the value of that you own is the size of your position. However, you do not need to own an asset to have a position in it of a specific size.
For example, when you have Short position, you borrow an asset and sell it, to buy it back cheaper at a later time. You do not own the asset, but you do have a position, the size of which equals the value of the underlying asset.
Another example when the amount of assets you actually own differs from the size of a position you have is when you use leverage. The borrowed funds enable you to open positions larger than the value of an asset that you own.
Finally, positions can be profitable if you can close them with a positive gain. And positions are losing if you can close them with a loss.