A "naive" person is someone who doesn't have much experience or understanding of life. Naive people are often gullible (meaning that they believe things that other people tell them). Some examples of people who are naive are

• a teenager who falls in love with her first boyfriend and thinks that they will stay together forever.
• someone who invests money in the stock market and loses it because they put all their money on one company.
• someone who supports a certain politician and believes his or her promises.

Being naive is a little different than being stupid. The word "stupid" has a more general meaning which includes not knowing things, but also not being able to think quickly or learn new things easily. The word "stupid" also has a more negative image than "naive".