Timelines can be best described as spans of time where important events took place within the Blazblue universe.(Ragna being sent back in time on end of CP Story Mode, being sent back to the time of the Black Beast. .  As time goes on, and the timelines weave together, so to say, they still remain completely seperate from each preceding timeline, in the sense that you are able to enter and exit each one at will, if you know how to do so.(Rachel sending Ragna back into the time of the Black Beast, then bringing him back to their exact present date again, at the end of the CP Story Mode)

Entering and Exiting a particular timeline requires that a character first travels through The Boundary(link), a strange plain connecting the timelines and worlds together in BB's Universe.(That Ragna part again) (Take-Mikazuchi being in a coccoon that was summoned by Sector Seven scientists in the BBCT intro)

After crossing timelines, a character will usually forget most of their present day/past day memories (depending on the direction they're traveling), but given enough time and hints they will return to them.(Rachel warning Ragna of this CP Story Mode. Then explain Ragna remembering stuff later as time went on.) This is a side-effect of traveling through the Boundary, rather than the actual switch of timelines itself.(Rachel talking about this to Ragna on the CP Story Mode.)

Some Important Timelines in the Blazeblue Universe Include:

The Black Beast is Born ( AD)

The Dark War ( AD - AD)

The Appearance of Noel Vermillion/The Destruction of Ibukido (AD)

The Ikaruga Civil War ( AD - AD)

Present Day ( AD - AD)

Timelines are also seperate from Arternate Worlds(link),(Makoto CSEX Story Mode, Rachel Alucard explaining to Makoto that she's entered a different world, and even a different timeline. pretty much should write everything RachelAlucard explains about the worlds vs. timelines here)  and you should be careful not to accidentially mix the two together. A character can be in the same world, but cross timelines -- they could be in a different world, and could be following the same timeline as the main world presently is, as well.