This amount is a return of capital from a trust that affect the adjusted cost base(ACB) of the mutual fund. This box is included for information and should not be needed for your tax return. For more information, take a look at the following CRA guide. The same CRA guide contains an example related to Box 42 of T3.
Here is another simpler example on how this amount can be used:
Year 0: you pay $1000 for fund X (starting ACB = $1000)
Year 1: you get a T3 Box 42: $30 (new ACB: 1000 - 30 = $970)
Year 2: you get a T3 Box 42: $25.5 (new ACB: 970 - 25.5 = 944.5)
Year 3: you sell fund X for a disposition of $1300 => the gain is 1300 - 944.5 = $355.5