The term ping has several meanings in Guild Wars.
Pinging the map
To ping the map or a location is to click on the radar which will radiate red rings and make a small ringing noise for the entire team. This is used for various purposes including giving teammates who are out of radar range an idea of which direction to go.
Pinging a skill bar or equipment
When someone asks you to ping your skill bar they mean to provide your equipped skills to your party. (See skill template for details on how this works.) Alternatively, this could also be a request to provide a template you want a teammate to use. This can be done similarly or by opening the load template dialog and selecting the option to send the template to chat.
The same thing can apply to equipment templates.
Pinging a status effect
Most frequently this is done by teammates who are either not communication on an external voice program or by team members who don't have microphones, CTRL clicking a status effect (enchantments, hexes, conditions, morale, etc) will inform your team that you have this status effect on you. Similarly, players may ping a particular skill being used by themselves or even foes to alert your team of its occurrence. Players can also ping health and energy by CTRL clicking the respective bars.