what is meant by poor shielding effect ?

Shielding effect can be defined as follows. 

In multi-electron atoms, the electrons present in the outermost shell do not experience the complete nuclear charge because of interaction with the inner electrons. Thus, the outermost electrons are shielded or screened from the nucleus by the inner electrons. This is known as shielding effect or screening effect. 

Poor shielding therefore means poor screening of nuclear charge. In other words, the nuclear charge is not effectively screened by electrons in question. 

1. - s orbitals have the largest screening effect for a given n value since s electrons are closer to the nucleus.
2. - p orbital's have the next highest screening effect and then comes d and then f orbital's. Thus in simple terms the screening effect decreases as

 s orbital's > p orbital's> d orbital's> f orbital's.

For example 5f orbital has poor shielding effect than 4f orbital.

This is because 5f orbitals are larger in size as compared to the 4f orbitals. They are more diffused in shape and therefore they can not shield nuclear charge effectively.