"autocommand BLAH" means: when the line is activated (by an "incoming" event"), execute the exec command BLAH.
What kind of event it takes to activate the line depends on how the line is configured.
If the line is configured for incoming modem control ("modem inout" or "modem dialin"), then the line will be activated when it sees its DSR signal go high. So if for example f the router line's DSR is cabled to a DTE's DTR, then the line will be activated when the DTE raises DTR.
If the line is not configured for modem control, then the line will be activated based upon incoming data. With "activation-character", you can specify a particular character receipt of which will make the line go active. (By default this is carriage return, 0x0d). Or with "no activation-character", the receipt of ANY character from the line will make the line go active.
Cheers,
Aaron