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Mirages are said to be illusions.
Literal
Mirage [topic-は] illusion is [quotative-と] is-said-[state-ている].
A short scientific/definitional statement. 蜃気楼 (しんきろう) literally 'clam-vapor-tower' — the name reflects an ancient folk belief that mirages were created by giant clams breathing out vapor. 幻影 (げんえい) is 'illusion, phantom, vision'. ~と言われている ('is said to be X') is the passive of 言う as a ~ている state — used to attribute the claim to general hearsay/consensus.