A natural language equivalence might help.
The following grammar
Sentence --> Sentence , Sentence | ϵ
Sentence --> She is a <adjective> girl
Sentence --> I am a <adjective> boy
Adjective --> good | bad
Can generate the sentence " She is a good girl, I am a bad boy"
A sentence is made up of phrases. Those phrases can be broken up into words.
That means , Non Terminal can derive into strings of Terminals and Non terminals.
The non terminals on the RHS are called phrases.
So,
E-->E+E,
E--> E-E
E--> a|b
is a phase structured grammar.
