When you resteem others content, you participate in a percentage of the curation pool that the post you resteemed has accumulated. The curation pool is a varyng percentage of the total upvote for each post. Authors (that's those that create posts) get the largest portion of the rewards pool because THEY make Steemit. Curation is very popular also because of the curation rewards pool for each post created. Simple guideline if you like what you see/read/hear/watch...upvote and at least think about resteeming.