Tamarind is a tropical "fruit". It's extremely sour. It's used in many tropical cuisines much like Lemon Juice is used in French cuisine.
Tamarinds are a "bean-like" pod. The pod is long, about 6" or longer, light brown, with a thin, hard "crust" that's a little thicker than paper. There are distinct bulges in the "pod" where there are seeds inside. Once you break open a pod, you will find the seeds (about twice the size of a Pente game piece), some long fibers, and a sticky, tar-like paste.
The paste is what you want to use.
To get the paste, break some of the pods open, immerse in water, and bring to a boil. Simmer for awhile (I can't remember how long) until the "paste" dissolves. Filter the mess to remove the seeds, fibers, and the covering of the pods. Bring the liquid back to a boil and reduce until thick. How long and how thick, I have no idea.
This is all from my memory of what I read out of either Thrill of the Grill, or License to Grill, both by Chris Schlesinger and Chris Willoughby.
I've never used Tamarind, but I have tasted it. It's EXTREMELY sour! I've seen the pods in my local Wally World, although that was about 12 years ago. I've seen the "bricks" of the rendered pulp/paste in my local Asian grocery store.