It's confusing for a lot of people at first. Every little thing you do costs RC (resource credits). Post, comments, upvotes, wallet transactions, etc all cost RC.
It's like a battery though and recharges over time. HP(Hive Power) gives you RC. Right now you have a delegation of 15 HP, which gave you roughly 30 Billion RC. I added 150 Billion so now your "battery" is bigger. You can still drain it but it'll take longer. Can still fully recharge. This is how transactions are free on this blockchain.
Once your post pays out in 6 days or so, you'll earn some HP of your own and that'll give you even more RC to use.
LOL I bet you're even more confused now! It'll make sense eventually.