models.ForeignKey() in django

Here is my code

from django.db import models
from django.utils import timezone
from django.contrib.auth.models import User

class Post(models.Model):
    title = models.CharField(max_length=100)
    content = models.TextField()
    date_posted = models.DateTimeField(default=timezone.now)
    author = models.ForeignKey(User, on_delete=models.CASCADE)

So I'm kinda confused with what models.DateTimeField(default=timezone.now) do, what does "default" mean here?

And I'm also confused what models.ForeignKey(User, on_delete=models.CASCADE) do, what is "on_delete=models.CASCADE" do and mean?

And is this code ( from django.contrib.auth.models ) a database for users?