It means exactly what it says. The "burden" or in other words "mission" would be for DREAMers to find solid concrete proof of when you arrived and how long you have been here. This is why everyone needs to find everything they can from when they were in the U.S. as a kid to now. This can be old photographs from recognizable landmarks, elementary/junior high/high school graduation diplomas, IDs, transcripts, etc. Immunization cards, health card IDs, everything that is SOLID proof.