Probing for some sandbox site knowledge

Hey There~
I am pretty new here and pathetically novice. Building a good ol' website is hands down a cake walk compared to this....BUT, I am up for the challenge and have learned an insane amount of back-end dynamics. My goal is to have an online marketplace using supporter and marketpress. While attempting (notice my humility) to plug in my marketpress settings, I discovered paypal chained payments (and the tedious process in which to properly establish such payment communication). During my descent into the abyss of utter web-dev-techy confusion, I came upon this term "sandbox payment testing". Can someone please translate in English?