1. when i create a structure i see a searchable field that i don't know where this is use.
2. is this possible that i add a field to article such as "sub titr" that this field searchable in articles without programming (in the other words, i want add some fields that can be searchable and showable in content)
thanks

Indeed that's what it's meant for. You can specify for each field whether liferay should index it and thereby show it in the results in case of a match.
Set them as not searchable and they the values won't be searched.