in addition to your main job

He makes a little money on the side by cleaning windows in his spare time. 他靠业余时间兼职清洗窗户赚些零用钱。

secretly

I think he has another woman on the side (= a relationship with a woman who is not his wife). 我认为他还有别的女人。

(of food in a restaurant) served on another plate, or on the side of the plate

I'd like a salad with the dressing on the side (= with the dressing served separately from the salad), please. 我要一盘色拉，调味汁单独盛盘，谢谢。

I'll have a omelette with fries on the side, please. 我要一个煎蛋，另要一盘炸薯条，谢谢。