InvestEngine reports “time-weighted rate of return”, as explained in this post: How does InvestEngine calculate returns?
The most straightforward method to calculate returns is “(end value minus start value) divided by start value”. This method would be valid if you buy all your ETFs on one particular day, and then look at your returns at a later date, with no transactions occurring in between.
But most of our portfolios are messier than that. Cash goes into and out of portfolios (through deposits, withdrawals and dividends) and ETFs get bought/sold at different times. The time-weighted return tries to account for this.