In Python, you can use nested loops to iterate over multiple sequences or nested data structures, such as lists of lists or dictionaries of lists. Nested loops are useful when you need to perform a repetitive operation on each combination of items from two or more sequences.

Here's an example of a nested loop that iterates over two lists and prints the combination of each item:

# Two lists to iterate over
colors = ['red', 'green', 'blue']
sizes = ['small', 'medium', 'large']
 
# Nested loop to print each combination of items
for color in colors:
    for size in sizes:
        print(color, size)

In this example, we define two lists called colors and sizes, then use a nested for loop to print each combination of items. The outer loop iterates over each color in the colors list, while the inner loop iterates over each size in the sizes list. The print() function is called with two arguments: the current color and size variables.

Nested loops can also be used with list comprehensions to generate a list of combinations. Here's an example that generates a list of all possible combinations of two numbers from 1 to 3:

# Generate a list of all possible combinations of two numbers from 1 to 3
combinations = [(i, j) for i in range(1, 4) for j in range(1, 4)]
 
# Print the list
print(combinations)

In this example, we use a list comprehension to generate a list called combinations. The comprehension includes two for loops that iterate over the range of numbers from 1 to 3, and generate all possible combinations of two numbers using a tuple. The resulting list contains nine tuples, each representing a unique combination of two numbers.