Means, "And a thumping good one". Or, that is was a really good one.
The ' (apostrophe) mean that a letter has been taken out. Rowling writes it like that to tell you how the sentence should be pronounced. So the apostrophe ( ' ) at the end of an' stands for the D in "and". The apostrophe ( ' ) at the end of thumpin' stands for the G in "thumping" (it's common to use a ' to signify dropping of the g sound").
"Good'un" is a little different, but it just like a contraction, such as won't (will not), it's (it is), or y'all (you all). When two words are combined like that, an apostrophe ( ' ) is used to represent the lost letters in a way. So "good'un" is just a contraction of "good" and "one". It's not a real word, but Rowling used this spelling of it so you would know Hagrid wasn't saying each letter, he puts them together when he talks.
Say each one out loud.
"and a thumping good one"
"an a thumpin goodun" (I removed the apostrophes to make it easier to read)
Can you hear the difference? It's just another way to show a character's personality and speech patterns, as well as making each character different them from each other.
Hope that helps!