It means to take 'liquid' steem, and lock it up, i.e. let it 'vest', for a period of 3 months. To the extent you 'vest' your steem, you have more steem power (SP). Having more SP allows you to have a greater impact when voting, and also allows you to earn more in curation rewards.

In essence, in exchange for you vesting resources within the steem blockchain, you gain more influence. That influence extends from voting on posts to voting for witnesses who secure the network.

Hope this helps! A wealth of resources exists here: https://steemit.com/welcome