A topic analysis will help you to clarify and understand what your assessment question is asking you to do.
You will generally be given three key pieces of information:
A hypothetical assessment question could be:
"Describe and critique the (dairy) farm system focusing on the following:
|Instruction words||Describe; critique|
|Qualifying words & phrases||animal husbandry; feedbase; viability|
|Key concepts||dairy farm;|
Write down the key concepts from your topic and have a go at brainstorming as many alternative keywords and phrases as possible.
Thinking about your topic in this way forces you to describe your topic in "other words", which will provide you with some useful keyword alternatives as well as help you to cement your understanding of the topic.
Dictionaries and encyclopaedias can help you to find additional keywords and get an overview of the concept.
Oxford Reference Online is a huge repository of subject-specific dictionaries, encyclopaedias, and other reference-type material.