map.put function calls that can be replaced with indexing operator ([]).
Using syntactic sugar makes your code simpler.
The quick-fix replaces put call with the assignment.
Example:
fun foo(map: MutableMap<Int, String>) {
map.put(42, "foo")
}
After the quick-fix is applied:
fun foo(map: MutableMap<Int, String>) {
map[42] = "foo"
}