Understanding Context Clues: Tension

In the heart of the forest, the trees stood tall, their branches swaying gently in the breeze. As the sun set, casting a golden glow, the sounds of chirping birds filled the air. The serenity of this place evoked a profound sense of calm. However, when a sudden rustle disturbed the peace, it created a moment of tension among the creatures of the woods. The sudden change in atmosphere made them alert.

What does the word tension most closely mean in this context?