Adjusted cost base (ACB) is the average cost of what you paid for a holding minus the fees. When you buy more at a different price later, then the average changes. When you sell later, the difference between the sale price and ACB is the capital gain or loss. So, if the investments are in a tax-exposed account like a corporate account or personal cash account, then it is needed to do your taxes. It does not matter for a TFSA, RESP, or RRSP since they are tax sheltered.
If you do need to track adjusted cost base, there are several approaches. You could simply give your accountant all of your investment statements or account history and they will calculate it for you. The problem arises if you don’t have statements or transactions going back to your first purchase. You accountant may also charge you more if it takes a lot of their time. There is also a free site that you can subscribe to and store/calculate ACB. You have to manually enter the transactions.
Tracking ACB yourself can be simple using Qtrade and an excel file that I made. You can import your transaction history into Excel, sort it, and copy/paste into the ACB Excel file.