Time

Time is a very complex matter. Normally it is very linear, one timeline, but if something is changed in the past unnaturally like through time travel, more strands come out of the one line. Those are alternate timelines.

Sometimes however, these things occur naturally. For example if something was very close to happen in a quick event, an alternate timeline could be born naturally during that event, yet nobody would know anything.

Time as an Energy
Time can be manipulated through energy, by slowing or speeding up time for the individual using it. The more control, the more it can be manipulated. Some can even use the energy to time travel, but it would never end well, as the Time Guardians would hunt the time traveler down.