since Para stabilizes rresonance forms better than Meta, it makes it a more stable anion, therefore more acidic. When you reonate an anion around the benzene ring, neg charge comes to the carbom bearing the EWG, which stabilizes this resonance form. However, when EWG is in the Meta position, negative charge never gets to this carbon and therefore this group does not get a chance to resonance stabilize it. He se, para is more acidic. Hope this helps.
Drawing resonance structures helps for this stuff. If you draw them, the negative charge will be pushed into the o or p positions & if you consider that there is an electron withdrawing group on there it'll stabilize it. Not sure if that is actually right, but makes sense.