Vegetables preserved in salt, such as mustard greens and radish: salty and strong, so a little goes a long way. It is eaten on its own with meals, or chopped and stir-fried with shredded pork or added to noodles.
For breakfast I like to have congee with pickled vegetables.
Tips
culture
咸菜 is a category, not one specific dish: any vegetable preserved in salt - most often Chinese mustard greens (雪里蕻, 雪菜), cabbage, radish, or cucumber. It's a staple side dish for breakfast 稀饭 (congee) and 馒头 (steamed buns), and a humble symbol of frugal home cooking. Compare 泡菜 (pàocài) which can mean either Sichuan brined vegetables or Korean kimchi.