A sidebar is a little piece from everyday life at the covenant of Mystikae Eikona. It is a chance to role-play a bit and give us more of your characters NOT within the context of bigger story arcs and subplots.
The basic rules are:
[i]A sidebar must be focused on one of your characters or one of the grogs.
You may not make other characters act but other characters may post within a sidebar and initiate action with you, at which point you and the other character may work through your encounter as you see fit.
You may address another character in a public place, or visit their sanctum (at your own risk) or work area.
You should probably send them a PM so they know you are interacting in a sidebar and can join in. If they don't you are perfectly within your rights to comment about how stuck up or pissy the other character is.
Anyone may interact with the character on which the sidebar is focused.
You may not affect the main storyline with your action (no quickly forging magic blades for those about to go out seeking the alleged murderers of the Redcap, for example--main storyline actions go in the Chapters, not in sidebars).
You may set a sidebar last week, last month, or last year but you cannot "give" yourself something you don't already have.[/i]
Basically a sidebar is just a way to have a little fun and do some character building. Have fun with it!