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."