Do they mean JUST jacket, Or Jacket AND TIE? 
Depends on the establishment. Out here in California, people think wearing a jacket over a tshirt is ok, and meets the dining requirements. YMMV.
If you have to wear a tie, I would expect that to be stated.
It is usually just a jacket and they sometimes have “loaner blazers” if you are with a party and the only one not meeting the requirement. I had that happen to me in New Orleans when my parents and their friends tried to take me out to dinner. The host allowed me to come in but he gave me a jacket that I had to wear. I had a button down shirt on so it looked fine. I have a feeling that if I had shorts or a t-shirt on, we would have all been turned away.
Nope. Excuse me, I’m going to pass the opportunity for a cheap joke to the next entity in line. 'Twasn’t my fault. The lady in front of me dodged off like a spooked squirrel. Never mind me. Hey! It’s your turn. Come back here, hey…dang.
They’d like you to wear ALL of the other clothes that you’d normally be wearing, plus a sport coat or suit jacket.
OK. “You can come in, but don’t start anything.”
RR
You mean I can’t go in dressed like Porky Pig?
The Magic Castle in Hollywood requires coat and tie, but states so clearly in their literature. So my thought matches with those above: jacket required means ties are optional.