The best answer is probably : “It is going to be tight”
It could mean that doing something before a given deadline will be very difficult. That you might not have the time to do something.

Or that it will be a tight game : “Les Red Sox de boston vont affronter les Yankees, ça sera tendu” “Boston Red Sox will face the Yankees, it’s going to be a very tight game.”

It can, in the same way, express a conflict between people :
“Je vais voir ma belle-mere, ça sera tendu” “I’m going to see my mother-in-law, it’s going to be tense”

In a totally litteral meaning, it just means it is going to be taut. “La corde sera tendue” “The rope will be taut”

The idea of tension, stiffness, tightness is predominant in this sentence.