Most often a dish made out of spinach which has been boiled for a long time, till it's only a paste. saag can also be made from other vegetables

Saag usually refers to spinach on the Indian restaurant menu although, strictly speaking, it means soft green leaves like fresh fenugreek leaves, mustard greens or, of course, spinach.

Spinach, but can also refer to other greens

Saag is a curry-like side dish eaten with Indian or Pakistani bread such as roti or naan. Saag can be made from spinach, mustard leaves, or other greens, along with added spices and sometimes other ingredients. A common form of saag is "Saag paneer", which contains a type of cheese known as paneer.