imagine: you should have finished your homework yesterday but you watched TV, today, when your teacher ask you why you didnt finish your homework , you say "because I was so tired" or "because I had to do other things" , then your teacher can say : dont "避重就轻"(不要避重就轻) it means your teacher believe that "tired" or "other things" is true(you dont lie) but this is not an important reason, you cant use this as your reason why you didnt finish your homework
OR we can say even if your were tired or you had to do other things but you could finish your homework its not a strong reason.
避:avoid
重:heavy ----> important
就:tend to
轻:light ----> trivial
attention: 就 means tend to
in old chinese, 就 just have one meaning: tend to
we can see some usages like this today: 就要 (be going to)
in current chinese, 就 means just too. 我就吃香蕉 I just eat banana.
but in old chinese, 就 just means tend to
there is another 成语: 行将就木
行将:就要 will or be going to (old chinese there is no 行将 in current chinese)
就:tend to or near to
木:wood ---->coffin (coffin is made of wood so we use 木 representing coffin)
so this 成语 means someone is going to die
example: 我已行将就木,没什么可怕的了。 I am going to die, nothing can make me feel scared but I am going to die comes from "我已行将就木" it means I am old
so we should translate like this: I am so old, nothing can make me feel scared
this is 就