It's a fairly confusing topic that i have trouble with as well. However, i have a technique that may work for you (it uses the principles of retrieval cues, but also ROTE learning to help.)
Unfortunately, you'll probably need to have a clear definition of the concept. My advice would be to memorise this one:
'The semantic network theory proposes that infomation is stored in a series of hierarchical nodes that associate to form meanings.'
After memorising that, there are four key aspects of the concept you'll need to know.
1. They are interrelated concepts and nodes.
2. Concepts are represented and connected by meaningful links.
3. The retrieval of one particular memory may activate other memories that are associated with it.
4. The stronger the link, the closer the meaning and the quicker the retrieval.
For these, remember these key words: Interrelated, meaningful links, activation, association between concepts.
I know this isn't really what you wanted, but i don't think there is a simple definition of the Semantic Network Theory lol. It's a concept that basically just needs to be ROTE learned, in my opinion.