Looting stands for finding things.

Looting is quite basic in the city. Just walk over to the ? sign and hold "F" for finding. If you go too far from the ? sign you are not able to loot.
Weapons, armours, money, medical kits, ammos, food and clothing could all be looted in the city.
TIP: When a zombie spotted you and was near you while you are looting, just walk around in circles with the zombie and hold "F" at the same time.

When you're looting and the item pops up zombies could not harm you. As long as the AFK sign written on the bottom is showing you could not be harmed.