Investment theory is about how to decide how to invest money to get the best return on your money. To do this, you must decide what kind of investments you want to make and how much of each type of investment you should buy.
For example, let's say you have $1000. You could invest it all in stocks, which are like small pieces of a company that you own. If the company does well, the value of your stocks will go up and you will make money. But if the company doesn't do well, the value of your stocks can go down and you can lose money.
You also could invest your money in bonds. These are like loans that you make to the government or a company. In exchange for lending them your money, the government or company will pay you interest.
Finally, you could invest in real estate, like buying a house or a piece of land. The value of the house or land could go up or down over time, but if you buy them for the right price, you could make money when you sell them.
So when deciding how to invest your money, you must decide how much risk you want to take and which type of investments you believe will do the best over time. It is not easy to decide where to invest your money, so it is important to do your research and to talk to an expert if you don't know what to do.