come in cielo così in terra.

Padre nostro, che sei nei cieli,
sia santificato il tuo nome,
venga il tuo regno, sia fatta la tua volontà,
come in cielo così in terra.

come

'come' means 'like' or 'as'.

così

'così' is a very handy word which means 'like this', 'like that', 'thus', 'so' or 'like so'.

in terra

As I've said before, prepositions are rather unpredictable. Whereas it looks as though this means 'in earth', obviously it means 'on earth'. Best not to worry about it too much.