Strong numbers indicate the numbers of the word roots in the original Greek and Hebrew Bible.
The advantage of searching by Strong numbers is that the conjugation and the words used in the given translation have no influence on the search as it always searches the Greek/Hebrew words belonging to the given Strong number.
When you use Strong numbers you can not use wildcard signs (e.g. *).
E.g.:
G5547
The results contain the word with Strong number G5547 which is Greek Khristos that is the word Christ, Messiah.
H4899
The results contain the word with Strong number H4899 which is the Hebrew word for Messiah.
G3563 AND NOT heart*
The results contain the word with Strong number G3563 which is the Greek word nus that is the sense of the heart but do not contain the word 'heart' and its conjugated forms.