What does it mean to be similar?

Basically the same, with slight differences.

The idiom, whose pinyin is dàtóngxiǎoyì, comes from Zhuangzi·Tianxia by Zhuang Zhou. Idiom usage: conjunction, as predicate: used for comparison.

Synonyms of similar but slightly different: almost the same, almost the same, about the same, exactly the same, half the same, seek common ground while reserving differences. Antonyms for similar: vastly different, vastly different, vastly different, as different as clouds and mud, as different as clouds and mud, vastly different. You both have similar opinions, so don't end up arguing.