The term "paper millionaire" means exactly as it sounds - someone who is a millionaire, but strictly on paper. Most of the shares these Twitter employees have

The term "paper millionaire" means exactly as it sounds - someone who is a millionaire, but strictly on paper. Most of the shares these Twitter employees have received cannot be sold until a certain amount of time. So the money is "on paper". They don't millions of dollars in their bank accounts - rather, they simply owns shares that are worth a million dollars.