GoldenGate info all command -- Understanding the output

Hi,

It would be great, if you could explain the output of the info all command and especially about the lag.  What does lag  and chekpoint mean for the below commands?

GGSCI (ddrtest3d) 1> info all

Program     Status      Group       Lag at Chkpt  Time Since Chkpt

MANAGER     RUNNING
EXTRACT     RUNNING     EXT_DDRQ    00:00:00      00:00:02
EXTRACT     RUNNING     PMP_DDRQ    00:00:00      00:00:05

GGSCI (ddrtest3d) 2> info EXT_DDRQ, detail

EXTRACT    EXT_DDRQ  Last Started 2013-06-05 11:56   Status RUNNING
Checkpoint Lag       00:00:00 (updated 00:00:08 ago)
Log Read Checkpoint  Oracle Redo Logs
                     2013-07-19 10:32:37  Seqno 36894, RBA 471864832
                     SCN 141.1937959651 (607528348387)

  Target Extract Trails:

  Remote Trail Name                                Seqno        RBA     Max MB

  /gghome/gg_EDW/product/11.2.0/dirdat/ddrq03/ex       6365    7695091        100