In computer programming, a guard is a boolean expression that must evaluate to true if the program execution is to continue in the branch in question (more here). They can replace the nested if-else statements for example, acting like the security guards: you can go through in a function only if you pass the conditions, otherwise just leave.

Some nested if-else statements could be like:

def analyzeInput(input):
    if i >= 0:
        if i <= 10:
            print "Do the calculation..."
        else:
            print "ERROR:The number should be less than 10"
    else:
        print "ERROR The number shouldn't be over 0"
 
    print "ERROR MSG: %s" % error
 
i = int(raw_input("Provide a number please between 1-10 "))
print i
analyzeInput(i)

Using guards, the function can be transformed into:

# refactoring the code using guards
def analyzeInputRef(input):
    if i < 0:
        print "ERROR:The number shouldn't be over 0"
        return 
 
    if i > 10:
        print "ERROR:The number should be less than 10"
        return
 
    print "Do the calculation..."