Translation:It is important for each one to look around.
That's weird because DL's hint shows it as one word, which of course is correct.
I'm having trouble understanding the use of "si" in many of the sentences in this lesson
In this sentence it is because the verb is reflexive: guardarsi. It is used in sentences like 'to look at himself in the mirror' ='guardarsi nello specchio'. To look around 'guardarsi intorno' actually means 'to look around herself/himself' but in spoken English we often drop the 'oneself/himself/themselves etc'. Good dictionaries usually put the reflexive form of a verb (if there is one) at the end of the entry, and will give you examples of how they are used. Good luck - it will make sense after a while - honestly!
It can also, in other examples, be the impersonal or passive One does something or something is done.
It means look around themselves ie a reflexive verb... which DL has just marked wrong. Here in UK we say' look around you' for' watch your back' yet they do not allow 'that everyone look around them' !!!!!