Nodes are what create a Logix program, they do various calculations or operations. Some nodes only run calculations based on their input values and provide the outputs readily, others are "passthrough" nodes that require an impulse to trigger or affect their operation. Often passthrough nodes perform a change in the datamodel, though this is not always the case.
Passthrough nodes can be chained together using the corresponding impulse passthrough outputs, to control the order of operations.
Passthrough nodes and other impulse-generating nodes, such as events, can have data outputs that only present valid data within their accompanying output impulse's Evaluation Context - You must process them using the same impulse that the node was triggered with or the data will be gone.