Portfolio
When you buy any stock/ETF, it gets delivered to your demat account on a T (transaction date) + 2 basis after the settlement is processed by the stock exchange. Everything that gets delivered to your demat account becomes a part of your portfolio. All such stocks/ETFs are accessible under Portfolio.
Positions
You may buy a stock and sell it on the same day, even before the delivery is met. This is called intraday trading. The stock sold even before delivery never reaches your demat account. We call such trades positions. For eg, we buy a stock with the intent to sell it the same day, we say that we have taken a position and we can close/square off this position even before the delivery will be met.