Adjusted Cost Base is the term that is used to describe the average cost of shares that an investor has bought. This is necessary so that you (and the government!) know how much tax you owe when you sell your shares. If you bought all of your shares at one time, there is nothing to it. If, however, you accumulated your shares through several separate purchases, you have some math in your future, in the form of the average cost method.
Doing the math
If you’re not thusly inclined, the mathematics of investing can quickly get ahead of you. While you (might!) argue that some calculations that investors routinely do are not absolutely necessary to tracking your investing progress, determining your adjusted cost base is not one of them; there is nothing the bit least optional about being able to determine how much you paid for your stocks. You need to know so you can calculate your taxes. Fortunately, it’s not that difficult. (The math part, not the paying taxes part; sorry, no help there.)