(someone's muscles) are tense

When something is squeezed tightly, you can describe it as "tense". Things that can be "tense" include:

• someone's muscles
• a rope that's being pulled in both directions

"Tense" muscles are hard from being squeezed tightly, usually because of stress.

You can also describe a person's facial expressions as "tense". It means that someone looks worried, stressed out, or concentrated. For example, this sentence might appear in a novel:

She bent closer, her expression tense with concentration.

Stressful or worrisome situations can be described as "tense", too.

A: How was it?

B: It was pretty tense. Jared yelled at some of the salespeople who hadn't hit their goals.