Think of a single infusion mash as steeping your grains at a very specific temperature for a determined length of time (usually an hour). That is, when steeping grains with an extract recipe, you want to keep the steeping liquid somewhere between 150 and 170F for 15-30 minutes. When mashing, you want to keep the mash as close to that target temperature as possible (here, 150F) for a longer period of time.
What makes it a single infusion is that you are infusing water into the mash once in order to hit your target temp. It differs from a step mash in which you add successive infusions of hot/boiling water to raise the mash temp in steps, then hold the temp at those steps for a period of time.
Welcome to the hobby, and welcome to the community!