Concurrent means happening at the same time, as in two movies showing at the same theater on the same weekend.
You might notice another adjective, current, in concurrent. While current refers to something that is happening right now, concurrent describes two or more things happening at the same time. A prisoner who is serving two concurrent five-year sentences will serve those prison terms together, meaning that he'll probably get out of jail in five years rather than ten. To help you remember the meaning of concurrent, imagine two parallel kayaks being carried down a river by the current at the same time.