ratio is how much you uploaded divided by how much you downloaded
the more you upload (seed) the more it will increase your ratio (basic maths)
in order to keep downloading stuff you need to upload at least 30% (0.3) of the total you have downloaded
e.g.: you have downloaded 3 torrents, 3 GB each; total downloaded is 9 GB and you need to upload at least 3 GB to have 0.3 ratio to be able to download new things
"how can I upload aka seed?" you may ask
simply leave the client running and people will download from you
here some FAQs and more info on torrenting