To ride in the front passenger seat of a car, as opposed to the back. The term comes from the days when a stagecoach driver would have someone ride next to him on the driver's seat, with a shotgun to fend off outlaws.

In its present usage, "riding shotgun" rarely involves wielding a firearm of any sort.

When Liz was fourteen,
she finally got to sit in the front seat
about six years after her first request

She rolled down the window and leaned one elbow out
far enough that she could feel the air rush by;
so her hair could catch the wind

On the same day her dad took her to DQ Brazier:
chili dogs and a couple of dip cones
She imagined the future

When she would be eighteen.
able to drive her own car around town
long after dark,
with both windows rolled down