Nice friends!

What does it mean by 'be my guest'

nice and easy this one!

'be my guest' is the English way of saying that a person is welcome and that hospitality is extended to them. that hospitality usually involves making one's resources available - so it is used in the sense of 'help yourself' - make free with my assets.

'mi casa e su casa' (apologies for spelling) 'my home is your home' is the rule of thumb for hospitality around the world.

"Be my guest" is sometimes used sarcastically when you really don't want someone to help himself.

I can't say that I've heard it used sarcastically - I would have thought 'ironically' would be nearer as there are no wounding words in the expression.