what's the meaning of entailment in RDF model theory?

In the part 3. Entailment in RDF MT it is said as following:
We say that I satisfies E if I(E)=true, and that a set S of expressions entails E 
if every interpretation which satisfies every member of S also satisfies E'. 
If {E} entails E' we will say that E entails E'.

question? what's the meaning of first E' in above paragraph? If every interpretation
which satisfies every member of S also satisfies E', how can get the conclusion that
S entails E? what is the relationship between E and E'? if E' is a subgraph of E, then
It can be an empty graph, So the above statement is nonsense.

If {E} entails E' we will say that E entails E'. what's the meaning of {E}, what does it stand for?
a set has only one element as E?
any one can explain it for me?