less important than related things

Her health is what matters - the cost of the treatment is of secondary importance. 她的健康是最重要的，治疗费用是其次的。

The need for secrecy is secondary to the need to take immediate action. 即刻行动起来比保守秘密更重要。

developing from something similar that existed earlier

The drug is not very effective against AIDS, though it may be used to treat secondary viral infections. 这药虽然可以用来治疗继发性病毒感染，但对治疗艾滋病本身不是很有效。

You can't just rely on secondary sources for your research into her life history - you ought to look at primary sources such as her letters and diaries. 你研究她的生平不能单靠二手资料，你应该查找原始资料，如她的信件和日记。

relating to the education of children approximately between the ages of 11 and 18 years old

secondary education 中等教育

They receive state money to train secondary teachers.