Gullibility is the quality of believing in things too quickly. Someone with a lot of gullibility can be easily tricked.
There's an old joke: "Did you know gullible isn't in the dictionary?" If you say "Really?" in response, then you are proving to be gullible: a little too ready to believe what you hear. The word for this quality or tendency is gullibility. Being superstitious shows gullibility. Believing tabloid newspapers shows gullibility. Falling easily for jokes and pranks shows gullibility. People with high gullibility are a little too trusting: they’re not suspicious of anyone.