When you have plenty of something, you have more than enough, or all that you want.
For example, if you have plenty of cupcakes at your birthday party, that means you have enough for everybody to have one. Maybe you'll even have some cupcakes left over! The cupcakes are plentiful.
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For another example, if you have plenty of time to do your homework, that means you have lots of time: all the time that you need to get it done. You might even have time left over to play.