Ratti (Hindi: रत्ती) is an Indian unit to measure small amounts of something, generally used for jewellery etc. It's a little less than a fifth of a gram, 0.182g to be precise. That's why it's used to signify something very small in poetry and songs.
Reza (Hindi: रेज़ा) is an Urdu word of Persian origin which means a particle, or a very small piece of something, often a gem.
Hence, the line ratti ratti reza reza jo hai tera le jaana means take away with you even the smallest thing that is yours, the smallest thing that you have given me.