You can buy a USD security in a CAD account and we will automatically convert CAD funds (FX) to cover the cost of the trade but with a mark-up from the Bank of Canada (BOC) spot rate. The opposite happens when you sell. There is a markdown from the BOC spot rate. This is just like shopping in the US using Canadian cash or credit card. The difference between the buy rate and the sell rate is called the spread.
When you buy and sell a USD security in a CAD account on the same day, we can wash the rates. But you are required to phone us before market close at 4pm ET to make this request. The result of this wash is, instead of charging the buy rate and sell rate on your trades, we would apply only one rate (either the buy or sell rate depending on which side is greater in dollar value). This saves you on paying the spread twice.
If you think you will do a lot of USD trading then you should consider a USD account. Note that USD TFSA, RRSP, and RRIF accounts have a $15 USD/quarter fee. But this will easily pay for itself by avoiding the FX spreads. There is no fee to have a USD Cash or Margin Account.