When writing code errors, warnings and notices can occur. I know the idea behind errors. I suppose a warning is there to inform you about something that can cause an error,
A notice is an advisory message meaning "You probably shouldn't be doing what you're doing, but I'll let you do it anyway"
A warning is a message saying "You are doing something wrong and it is very likely to cause errors in the future, so please fix it."
Both notices and warnings will not halt execution of your script, although I would encourage you to take them seriously and strive to have not even one notice in your apps.