According to Steemit FAQ;
Steem Power (SP) is a measurement of how much influence a user has in the Steem network.
i.e The more SP you have, the more money you give to authors with an Upvote as well as the greater the curation rewards you can earn.
A lot has been said by @Kevinwong on why you should consider accumulating SP as well as on 10 Amazing Reasons to Invest into Steem Power by 
There are several ways by which you can acquire SP:
#1 BY EARNING AS A REWARD
SP can be earned as a reward for:
- Posting creative contents
- Upvoting other Steemians Posts (Curation)
#2 BY DELEGATION FROM ANOTHER STEEMIAN
SP can also be gotte via delegation from one Steemian to another as a means of assistance or encouragement in order to increase the influence of the receiver.
However, it should be Noted that:
- The Delegator remains in full control of the delegated SP and can take back a delegation at any time.
- Delegated SP cannot be powered down, neither by the Delegator nor by the receiver.
#3 BY BUYING SP WITH OTHER CRYPTOCURRENCIES
SP can also be bought with other cryptocurrencies like Dogecoin, Bitcoin, Litecoin etc.
While the first two means of acquiring SP (Posting and Delegation) are the most common, I want to emphasize more on the third, i.e
BUYING SP WITH OTHER CRYPTOCURRENCIES
I know you would have been thinking about the financial cost of buying SP with other Cryptocurrencies.
Well, I'll be sharing with you what I've been doing in order to achieve this without a single penny, probably if you can follow suit, it can be of help to you.
There are some Cryptocurrency websites whereby you can claim and accumulate Cryptocoins and use them to buy SP here on Steemit.
Among the ones that I've used and never failed include the following:
#1 COINPOT
Coinpot is a cryptocurrency wallet with 5 different cryptocurrencies ( Bitcoin Core, Bitcoin Cash, Dogecoin, Litecoin, Dashcoin).
The beauty of coinpot is that it allows conversion of one coin to another, thereby allowing accumulation of a particular coin of choice which can be used to buy SP.
Below are the STEPS to follow in order to claim cryptocoins on coinpot
- Register on Coinpot
- Sign in to these 5 Faucets with the same Email address you used for registering coinpot Register on these 2 faucets with the same Email Address
- Start claiming these coins as many times as you can daily
Here is the screenshot of what I've claimed over the period of 4 days
#2 FREEDOGECOIN
This is another reliable site whereby you can claim free Dogecoin at intervals 60mins,
All you have to do here is to Register and start claiming dogecoin .
#3 FREEBITCOIN
Just exactly like Freedogecoin, all you need is to Register and start claiming free Bitcoin at intervals of 60mins.
All these requires nothing but your commitment and in no time you would have accumulated enough coins that you can use to increase your SP thereby increasing your earning rate on Steemit.