Assistance is a property of instruction. Intuitively, assistance refers to how much help the learner gets from the instruction as they are learning. The term scaffolding is nearly synonymous.
Operationally, assistance is equated with the probability of a good performance during training. For instance, if the training involves solving a multistep problem, and the experimental instruction causes students to have a higher probability of correctly entering steps than the control instruction, then the experimental instruction has a higher degree of assistance.
It is important to note, that saying students get more instructional assistance from one instructional approach than another does not mean students will learn more. Assistance is not equal to learning. Sometimes less assistance is better, for instance, because it may more engaging or because it may require more deep thought. See the Assistance Hypothesis and the assistance dilemma.