A portfolio is a collection of like things. A photographer's portfolio is a compilation of photos they've taken. A graphic designer's portfolio is full of designs they've done from logos to brand identities, to websites, and illustrations. A portfolio is used to show the breadth and depth of whatever it is showcasing.
An investment portfolio is the entire collection of all your investments.
And just for good measure, let's review the definition of an investment. From an economic standpoint, an investment is when you purchase goods for the future purpose of creating wealth. From a finance perspective, investments are monetary assets bought to produce income or make a profit by selling the asset in the future at a higher price.