Set the weight

Rule of thumb with a deadfall is you set the weight five times that of the target animal

What does set the weight mean?

you are talking about a kind of trap that has a deadfall - something falls and lifts the trapped animal into the air.
The thing that falls (the weight) should be about 5 times the weight of the target animal. So you have to “set the weight” (i.e. choose the thing that will fall) appropriately.