Hello SplinterMates,
I'm starting a SplinterLands Q&A [Question & Answer] Series; where I'll be answering some of the common questions/doubts that players have while playing the game.
I got this idea from a friend of mine 
KayKay had some questions about the game and was finding it difficult to get the answers. So as to help him along with other players - I thought, I'll start a SplinterLands Q&A Series..
Question1::
I have bought Summoner Card but I'm not able to use in battle - Why?
Answer1::
In order to answer this question - You need to first understand what is a card cool-down period. Where you can see whether the card is on a cool-down period or not.
Let's have a look at an example to understand - What is this card cool-down all about?
Following is the snapshot of Summoner LYANNA NATURA
If you closely look at Watch Symbol - That should give you an indication whether a card is on a cool-down period or not. In our example Summoner LYANNA NATURA Level 3 Cards which is being sold for $6.59 is at a cool-down period coz. the player would have played with this card in Ranked Matches before having it put for sale.
Just hover your mouse over the Watch Symbol and you will notice the following text - Which says -
"This card was recently used in battle will not able to be played in Ranked matches for 4 Days"
Now if you still go on to purchase LYANNA NATURA level 3 Card - then only after 4 days you will be able to use her in the battle.Now that you have some background with regards to What a cool-down period of a card means. Let's answer the main question which was:-
The answer is pretty simple - If you bought a card which was on a cool-down period then you will need to wait once the cool-down period finishes and when it does, you should be able to play with your card in the battles moving forward.
A Pro Tip While Buying Cards::
Always look at the Watch Symbol ; when you are buying the card & until it's very necessary and you can await then only go for buying the card which has a cool-down waiting time. Else try avoiding it - If you plan to use the card immediately..
I hope this information will be very helpful to new players like my friend Kaykay and in near future should act as a guide to their queries around the SplinterLands game.
Do you love playing games and do you want to earn as well while playing the game. If YES then come join this amazing online trading cards game SPLINTERLANDS here
Until then See You in my Next::
PS:- All the images are taken from the Splinterlands Website