To let saliva or spit come out of your mouth.
Slobbering is usually seen as not very nice or polite.
Examples:
The baby will slobber when it's teething.
The dog slobbers when it's excited.
Please don't slobber on the table.
The baby slobbered all over his shirt.
The dog slobbered so much, it made a big puddle.
She slobbered all over her food, like a messy eater.