What does this Romaji mean in English?

I suppose they're trying to say they're in love with you; however, it's not correct.

Anata - you

wa - topic marker

Ai - love

ni - in (preposition)

aru - to have; exist; there is/are

Would be correct if they said:

（あなたが）恋をしている。

(Anata ga) koi wo shiteiru

*The syllables in the parentheses can be omitted if the context is understood by the audience. So the speaker could just say "koi wo shiteiru" and from context, the listener would understand "I'm in love with you."

Would be even better if they said:

（あなたが）好きです。

(Anata ga) suki desu.

or

（あなたが）大好きです。

(Anata ga) daisuki desu.

*said when you're not seriously, romantically involved.

Which means "I (really) like you," which can be understood as "I love you."

Iced pie with slurry with minced bojangles. Odd but true schmoo.