"Mari" is basically a soft imperative word, which transforms a sentence into its politer version. This nuance of politeness is hard to portray in English, so I'll use its French equivalent: Ayo pergi! = Allons! Mari pergi! = Allons-y! Ayo makan! = Manges! Mari makan! = Mangez-vous / Mangeons-nous! As you notice, the difference between "ayo" and "mari" are like using the "tutoyer" and "vouvoyer" versions of imperative (though the "mari" in Indonesian can sometimes be used to include first-person plural pronoun too).

ayo, mari - come, come on, let's go "let's do something together" "or "invitation to do something together" Makan! = Eat! Mari makan - come on, let's eat. ( a nicer softer way to say it) Mari kita cuba - let us try. Mari kita lihat - let us see. Mari tepuk tangan - let us clap. You might also hear, Kemari - Come here. Mari - shorter form of kemari. So if someone say's Mari, Mari. They can either mean Come here, come here or come, come ( a shorten invitation , come let's eat)