What is the meaning of undecidability in Rice Theorem?
Undecidable means not decidable. Undecidable problems may or may not be semi-decidable.

To see that an undecidable problem is not necessarily semi-decidable, observe that there are uncountably many problems but, since each decidable or semi-decidable problem corresponds to a Turing machine, there are only countably many decidable and semi-decidable problems.