Specifically, the tension comes from the fact that you are in some fictional former soviet state, which just opened up a border security crossing again. You've been posted to man it, and now live with your family in a government housing estate. You have a finite amount of time to process people through the security gate each 'day' and you're paid based on how many people you get through, with fines for incorrect processing (which of course gets harder and more convoluted as the game goes on).
At the end of each day, your income and expenses (for rent, food, electricity, medicine for your son/wife/elderly parents as needed) are shown. The tension comes from rushing to process enough people to ensure you don't have to go without food, or turn off electricity, or not medicate your family. However, if you make too many mistakes by rushing, you get fined too much and you don't get to care for your family.
It's really far more gripping than you'd expect because the whole art style, audio and interactions (both between days via a daily newspaper, and with the people as they enter through your booth/events that unfold during that) really help paint a picture of a desolate former soviet wasteland of poverty and misery. Couple that with the distilled essence of 'putting food on the table for your family so they don't starve' being the motivating factor, and it can be quite high-tension to try to not have your family inevitably starve and freeze to death with crippling illness.
For $5 (if you grab it during a steam sale), it's worth the experience for sure. I'd have been happy paying the standard $10 for the unique experience it offers.