I can't tell you what it really is
I can only tell you what it feels like
The “it” Em is referring to here is love. This song is about love more or less, and I can tell that he’s not lying because you’d have to be in love to come up with some of the corny lines he’s about to unleash
Eminem’s love is too deep a matter for him to comprehend rationally; he can only talk about his emotional reaction (pain, sadness), which are based in perception rather than definable reality. Thus, his song is subjective, just one man’s interpretation, and this line is a sort of disclaimer.
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