It is used to introduce a contrasting statement or idea, often after a statement that seems to contradict the following one.
Here are some examples:
* Il fait froid, pourtant j'aime l'hiver. (It's cold, but I like winter.)
* Elle est malade, pourtant elle va travailler. (She's sick, but she's going to work.)
* Le film était long, pourtant j'ai trouvé l'histoire captivante. (The movie was long, but I found the story captivating.)
"Pourtant" is a conjunction that can be placed at the beginning or middle of a sentence.
It's a useful word to know when you want to express a contrast or a contradiction in French.