Means: I will do a handjob for you. And "sich einen runterholen" stands for (male) masturbation. Both are slang terms.
"Du" is written with a capital "D" because in the old spelling - which many, many people still use - Du/Ihr/Sie etc. is always written with a capital letter. And the apostroph just indicates that the word is shortened, because the term is "ich hole".
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