Resteem it's just like share on other social media. You share/resteems posts that you liked so that your followers will read them too. Sometimes I maybe loose with my upvotes , I have enough SP to do so , but with my resteems I am strict and share only posts that I like a lot and totally agree.
Hope it helped . If you have any other questions don't hesitate to ask me .
Steem on !!! :)