If you are new here or you have a very little of SP staked you should be very cautious of your resource credits. Steemit is basically a dapp on top of the STEEM blockchain and every post you write, every upvote you give and every comment you do is forever saved in this blockchain.
That sounds amazing, but how can STEEM handle all the spam? Every operation you do costs some amount of resource credits you own.
You can power up your STEEM, which not only gives you stronger upvote, but also alocate to you a bigger share of the existing resource credits for you to use.
There is this amazing site - https://steem.supply/@yourusername, where you can find how many posts, upvotes and transfers you can do till you have insufficient amount of RC. Don't worry, they are regenerated every day. I must say though that being a new user on steemit is not easy at all as you really have to be deciding if it's better to write a post or upvote a couple of posts.