I always hear people talking bout high resolution model and a low resolution model but my question is: What do they mean with that and how can I make it a high-res model and low-res ???
Thanks for your help:)
Hi-res = many polygons = many details + smooth curves + slow to render
Lo-res = few polygons = few details + blocky + fast to render
Usually lo-res models are used for games, as these models have to be animated at realtime, whereas hires models are used in cg images and movies, since you have much time to render them.
For computer games it is a common technique to create a hi-res and a lo-res model and then create a ‘normal map’ from the hi-res model, which is projected back onto the lo-res model. This way it is possible to make lo-res models look almost as detailed as hi-res models.
To create lo-res models, just model them and pay attention to the polygon-count. 
For hi-res models usually ‘subdivision surfaces’ are use. A rather low resolution model is subdivided into more polygons, to make it look smooth.
That’s not everything, but I guess it’s enough to get a basics idea of the two terms.