That information is contained in "
:help :buffers". I will quote it here:
Indicators (chars in the same column are mutually exclusive):
u an unlisted buffer (only displayed when [!] is used)
% the buffer in the current window
# the alternate buffer for ":e #" and CTRL-^
a an active buffer: it is loaded and visible
h a hidden buffer: It is loaded, but currently not
displayed in a window |hidden-buffer|
- a buffer with 'modifiable' off
= a readonly buffer
+ a modified buffer
x a buffer with read errors
So to answer your specific question, "%" means the current buffer in the window your cursor is in when you did "
:buffers". "a" means a buffer you can currently see. Finally, "#" is the alternate buffer, which usually means the last buffer you were editing.