what do they mean by u had me at hello ?

It means they lack the imagination to say anything original, so they steal a cliched line from a Tom Cruise movie

It means they already liked you/ were sold when you said hello.

It means that you had their attention before you said hello. They liked you from the moment they saw you.

And all the other stuff they said since was not necessary and may even start to be detrimental if continued.

You had me at, "what do they mean by u had me at hello." ;)

they liked you immidietly

I have no clue 👀😂

It means they disliked what you said after hello and now regard you as an idiot or bitch.

[youtube=https://www.youtube.com/watch?v=nhNUBG40bwE]