"That they may keep you from an adulteress, from the foreigner who flatters with her words"
The word foreigner is the Hebrew word nokri which means that which is foreign or strange. The word translated adulteress is the Hebrew word zur which is stranger. This is clearly a reference to the practice of allowing foreign women to ply their trade as prostitutes within a Jewish city. It also would suggest that everyone except the person you are to marry is strange or a foreigner to you.
Many times young people in a search for love will take any kind of love just to have some comfort. This is unfortunately a misguided action and only results in difficulty and pain. The proverb here reminds us yet again that wisdom and understanding will keep you from the person who is foreign to you. Seek the Lord and His choices for you
The idea of a strange man or woman has considerable potency for singles. I have asked men and women whether they feel that the person they are presently dating is really the right person for them. Countless numbers will say no; they are just right for right now. In other words, this is a person they are putting up with so they can get some love and companionship while they are waiting for Mr. or Ms. Right. Often this Mr. or Ms. Right now becomes their life companion because they did not use wisdom and understanding to distance themselves from a person who was not really the right mate for them.