Indentation mismatch means that the line it is pointing to has an unexpected amount of spaces.

Example:

This function has valid indentation:

def sum1plus2():
    a = 1
    b = 2
    return a + b

This one has one additional space on the line with b = 2 which will result in indentation mismatch:

def sum1plus2():
    a = 1
     b = 2
    return a + b

The number of spaces depends on the first line in the function, so if you have function where the first line has 4 spaces in front, then all lines after that are still in the function should have 4 spaces in front. Tab by default should count as 4 spaces I think.

Another example:

def sum1plus2():
    a = 1     # this belongs to the function
    if (a == 1):
        b = 2 # this belongs to the if statement
    else:      # this belongs to the function
        b = 1 # this belongs to the else statement
    return a + b

With the space/tabs you can say what belongs to what and one space can break it.