What it means "String in phython are immutable"?

Write the output for the following if strl = "THIRIKKURAL"
(i) print (strl [0])
(ii) print (strl [0:5])
(ill) print (strl [:5])
(iv) Print (strl [6:])

What has to be filled in the blank to get the following output
(i) Welcome python
(ii) Welcome to learn python From
(a) print ("Welcome" ____ "Python") strl = "Welcome"
(b) print (Strl ____ "to learn python")

What is meant by stride?

Write the output for the following statement
strl = "Welcome to learn python"
print (strl [: : - 2])