order string: smart

how do i use this order string? what does it do?

This order is issued when players use the right mouse click. It has different effects depending on how it's used (on the ground=move, on an enemy=attack).

Does doing AI need this smart action?

Doing AI? I'm not sure I'm following you, can you explain a bit more what you mean?

I went thru some AI page at wc3search, i saw some people saying about "smart" order string is good for AI, especially when you want to make 1 AI that act "smart". Is this true?

As much as your AI is like a super random AI that just issues smart orders on random units in the map , yes.

Otherwise your AI should already know that it is going to target an enemy so you could just use "attack"