If there's something you want, don't hesitate to say so.
Literal
Want [attributive-の] things [subject-が] exist-[if-ば] hesitation-without say.
~があれば ('if there is X') is the ~ば conditional on ある. 遠慮 ('reserve / hesitation / holding back') is a core Japanese social concept — the impulse to avoid imposing on others. 遠慮なく ('without reserve / don't hold back') is the standard way to tell someone they're welcome to ask freely.