To answer your question, we use this word when talking about people when we feel they believe everything they hear, or take things at face value, or can easily be convinced to do anything:
A: He said he was going to leave his wife and marry me!
B: That was five years ago and they're still together. Don't be so
It's not always a reproach - sometimes it just means innocent and unsuspecting:
She entered college a mousy, naive girl, and came out a confident,
It is also sometimes used to describe someone's actions, who doesn't suspect anything is amiss in a given situation:
"They made up a profile for a girl named Debbie on a dating site.
"Debbie" sent me her email address and I wrote her several times to
ask her out. They must have gotten a lot of laughs from my naive
attempts to meet her."