Socialization is the process by which we are taught the values of our society and its assumptions about how we are supposed to behave. We can be taught these things intentionally, or we can be taught them simply by what is implied.
An example of intentional socialization comes when parents explicitly tell their children what is expected of them. They might say "boys don't hit girls." Or they might say "big boys don't cry." These are values and norms that are being transmitted explicitly and intentionally.
However, we are also taught things by people who do not know that they are teaching us or who do not mean to be teaching us. When little girls are told how cute they look, people are not trying to teach them a lesson. But the girls are getting the message that looking cute is a good thing. When Disney movies show a female character waiting for her prince to rescue her, they are not trying to send a message. But little girls absorb the message that girls are meant to be passive.
In these ways, we are socialized both intentionally by people who are actively trying to teach us things and unintentionally by people who are simply expressing the values of our society as a matter of course.