To be tense means nervous or worried and unable to relax, or physically tight and stiff. I felt tense before my job interview, so my jaw stayed clenched.
Compared with nervous, which names the worry itself, tense suggests the whole body going tight and wound-up.
Two quick pictures
After a long day hunched over a desk, your neck muscles feel tight and stiff. That is being tense in the body sense. By evening my neck was so tense that turning my head hurt.
Or picture the last minute of a close game, when everyone is quiet and nobody can relax. The moment itself is tense. There were some tense moments in the second half of the game.
A person can be tense or a situation can be. I was tense means you felt nervous; the meeting was tense means the room felt that way for everyone in it.
A room, or even a game, can feel tense even when nobody says a word. The meeting was tense means everyone in the room felt nervous and worried, even if no single person said so. You can often sense it from silences, short answers, or the way people watch each other rather than from anything spoken.
That is why tense is a handy word for two related things: the nervous feeling in your mind, and the tightness in your body that often comes with it. Your shoulders or jaw can be tense even on a calm day, just from hunching over a desk.