When you become able to move again, you really appreciate the value of good health, don't you.
Literal
Move-able become-[when-と] health [possessive-の] preciousness [object-を] truly-feel [agreement-ますよね].
動けるようになる 'to become able to move' — potential form + ようになる (change of state). 有難さ 'preciousness, value' — the noun form of ありがたい 'grateful, precious.' 実感する 'to truly feel, to appreciate viscerally.' A reflection on health that resonates with anyone who's been ill.