Steemit Voting Power

Hello all

I am new to the steemit community. I am trying to contribute as much as I can to this community through active participation. However, I have observed that sometimes my upvotes have some voting power and sometimes they don't. I read from various blogs on steemit that a person has a limited steemit power during day and he gets 20 voting power each day added to his account. Can someone explain this in detail? Do we get new voting power added each day to our account in addition to the voting power we already have, for example my current voting power is 15, so do I get 15 votes everyday added to my account? It is said that we should not upvote all posts as that will make our voting power to zero. So how does our voting power REGENERATE?