This is usually caused if you have 2-factor authentication turned on for your Gmail account. You have to turn it off.

its just because i don't understand the 2-factor authentication turned your talking about

Google has a good tutorial on their web site explaining 2-factor (2-step) authentication.

You have to disable 2-factor in order to connect to your gmail account with SMTP. This is why I say in the seminar to use a 2nd gmail account for your mail sending - so you're not making your primary account at risk.