Idiom/chengyu meaning: 小葱拌豆腐一清二白
一清二白 is a very common idiom. It's well defined in this dictionary:

◎ 一清二白 yī qīng èr bái
(1) [be perfectly spotless]∶非常清白 e.g. 素来一清二白
(2) [be perfectly clear; as
clear as daylight]∶ 同“一清二楚” e.g. 记得一清二白

小葱拌豆腐一清二白 is 歇后语 as the other answer indicated. 歇后语 usually have two parts. The first part describes a way how the second part is generated.
In this case, 小葱拌豆腐 is a type of Chinese dish, which only consists of 小葱 and 豆腐. 青 and 白 refers to colors. 拌(stir) is the way the dish is made. 豆腐 is white(白). 葱 is cyan or green(青).
So, when you look at the dish, you would easily tell 小葱 from 豆腐（and vice versa） because their colors are very distinctive. One is 青(一青), and the other is 白（二白）. That's how 一青二白 forms. 一青二白 and 一清二白 are homophones. In spoken, you wouldn't hear any difference.
歇后语 is kind of word play in my view and usually used when one tries to make some funny/humorous effect, impress his listeners, etc. E.g. 做人要像小葱拌豆腐一样，一清二白.