Something that happens now and then occurs every once in a while. You call your grandma every single week, but your sibling only talks to her now and then.
You can use the phrase now and then for things you do occasionally or every so often. If your family almost always eats pizza on Friday night but sometimes orders Thai food instead, that means you get to eat your favorite pad Thai now and then. Now and then has been used this way since at least the 1550s — before that, people said then and then instead.