So let's pretend that your dad is a baker, and sends you to school every day with a slice of cake in your lunchbox. It's really good. You can trade that cake with the other kids, and because slices of cake are rare, everyone is willing to make good trades for that cake. They will do your homework for that slice of cake. They will give you a cool comic for that slice of cake. One slice of cake a day makes you the most important person on the playground.
So you decide that you're going to bring a WHOLE cake to school next time, so that you'll get even MORE things! But... now that cake isn't as rare, and everyone can tell that's the case, they are less willing to do the same work for just one slice. Now they know that they could ask for TWO slices of cake for doing your homework, or THREE slices for giving you that comic, and that they could realistically get it.
Having more cake available made each slice of cake less valuable. Now, instead of one slice of cake getting you homework, everyone is asking for two slices.
The same basic concept applies to money. Money is valuable because it is rare.
If there are only $1000 dollars out there, having $100 is a BIG deal! You've got 1/10th of all the money out there! That means you have a lot of power! But if the government decides to print more money, so now there's $1,000,000 dollars out there, only having $100 is no longer that special. The money you have isn't as valuable anymore.
That's inflation. When more money is printed, it loses its value. Many countries have tried this, thinking that the short term benefit would be worth it, but in the long run it has ruined many economies.