It means "I can't get my money out." It's stuck. It's frozen. It's "tied up." It's invested in an "illiquid" investment. Let's say you have $1,000,000. Let's say you use it to buy an apartment building and rent it out. Now you have $0 in cash and an apartment building that's worth $1,000,000. Someone comes to you and says "Could you donate $50,000 to our charity?" You say "No." They say "but you're rich!" You say "But I don't have any cash. All my money is tied up in real estate."

It means that you've spent that money on real estate. Maybe that person spent money purchasing houses or property. Now he/she can't spend that money because it's already spent or "tied up".

Dan Smith and Sindy have given you good answers. I will add that there is a saying (at least in the U.S.) "I'm house rich and cash poor."

It means that person has money invested in real estate.