What does Benjamin Franklin mean by this?: "Tart words make no friends; a spoonful of honey will catch more flies than a gallon of vinegar."

What this saying, repeated by Benjamin Franklin, means is
that it is easier to get people to do what you want by being nice than by being
mean.

When Franklin uses the phrase "tart words" he means
words that are angry.  He says that if you use angry words when talking to people they
will not want to be your friend.

Instead, he says, you have
to use honey.  When he says this, he mans that you have to use nice ("sweet") words and
actions.  If you act with a little kindness, it will be more effective than doing or
saying many angry things (the gallon of vinegar).

In this
saying, Franklin is using sour things as a metaphor for anger and sweet things as a
metaphor for kindness and nice actions.  He is saying that nice words and actions are
more effective than angry ones.