phobia vs simple phobia

phobia and simple phobia both are nouns.

Word	Noun	Adjective	Verb	Adverb
phobia	Yes	No	No	No
simple phobia	Yes	No	No	No

As nouns, simple phobia is a hyponym of phobia; that is, simple phobia is a word with a more specific, narrower meaning than phobia:

phobia: an anxiety disorder characterized by extreme and irrational fear of simple things or social situations
simple phobia: any phobia (other than agoraphobia) associated with relatively simple well-defined stimuli

Other hyponyms of phobia include agoraphobia, social phobia.

phobia (noun)	simple phobia (noun)
an anxiety disorder characterized by extreme and irrational fear of simple things or social situations	any phobia (other than agoraphobia) associated with relatively simple well-defined stimuli