@Husnain Bustam
Yes. Running balancer will start moving blocks from nodes where you have higher number of blocks to nodes which have less number of blocks. This depends on a number of factors. For example, you likely have balancing threshold set to 10% which means blocks can be distributed within the cluster within 10% of each other (one node has 10 blocks and other 9 or 11 would be acceptable - no need to balance further).
When balancer runs, it takes up network resources. You want to make sure you are not doing it when you have heavy load as those jobs will be affected. There are also checks against moving blocks as you may have current jobs using them.
check for following settings and how to change them for your needs.
dfs.balance.bandwidthPerSec (network bandwidth you want to assign to balancer)
dfs.datanode.balance.max.concurrent.moves (how many blocks you want to move concurrently).
You should check following thread. The accepted answer has good suggestion on how to start and have good performance.
https://community.hortonworks.com/questions/49959/even-when-i-ran-balancer-load-one-data-node-is-84....
