One of the features strings have is the ability to be used as a template. There are several different ways of formatting a string, and for the full list of possibilities, I encourage you to look up the documentation. Here are some common examples:

>>> greet_old = 'Hello %s!'
>>> greet_old % 'Fabrizio'
'Hello Fabrizio!'

>>> greet_positional = 'Hello {} {}!'
>>> greet_positional.format('Fabrizio', 'Romano')
'Hello Fabrizio Romano!'

>>> greet_positional_idx = 'This is {0}! {1} loves {0}!'
>>> greet_positional_idx.format('Python', 'Fabrizio')
'This is Python! Fabrizio loves Python!'
>>> greet_positional_idx.format('Coffee', 'Fab')
'This is Coffee! Fab loves Coffee!'

>>> keyword = 'Hello, my name is {name} {last_name}'
>>> keyword.format(name='Fabrizio', last_name='Romano')
'Hello, my name is Fabrizio Romano'

In the previous example, you can see four different ways of formatting stings. The first one, which relies on the % operator, is deprecated and shouldn't be used any more. The current, modern way to format a string is by using the format string method. You can see, from the different examples, that a pair of curly braces acts as a placeholder within the string. When we call format, we feed it data that replaces the placeholders. We can specify indexes (and much more) within the curly braces, and even names, which implies we'll have to call format using keyword arguments instead of positional ones.

Notice how greet_positional_idx is rendered differently by feeding different data to the call to format. Apparently, I'm into Python and coffee... big surprise!

One last feature I want to show you is a relatively new addition to Python (Version 3.6) and it's called formatted string literals. This feature is quite cool: strings are prefixed with f, and contain replacement fields surrounded by curly braces. Replacement fields are expressions evaluated at runtime, and then formatted using the format protocol:

>>> name = 'Fab'
>>> age = 42
>>> f"Hello! My name is {name} and I'm {age}"
"Hello! My name is Fab and I'm 42"
>>> from math import pi
>>> f"No arguing with {pi}, it's irrational..."
"No arguing with 3.141592653589793, it's irrational..."

Check out the official documentation to learn everything about string formatting and how powerful it can be.