I’m not quite sure where your confusion comes from, but a bollard is just a sturdy post. For the purposes of forming a barrier against heavy vehicles they’d be fixed in place and usually relatively strong.

There’s different kinds. some are weaker/lighter and just meant to make it difficult to accidentally drive into a pedestrian area, basically the same function as a curb but a little stronger or where you don’t want a step up/down for the pedestrians. Others are quite able to stop even heavy trucks.

The other kind you mentioned are probably rising bollards, meant to function as gates or to allow only certain types of vehicles (often buses) to pass.

You should probably assume somebody is talking about fixed bollards unless they explicitly mention retractable.

Understandable, some of the sites I'm describing have a combination of more than one you're talking about.

Bollards are the sidewalk kind, all metal or short metal-cored concrete poles, optionally also used to protect building corners and natural gas meters on properties with vehicle traffic. The kind that pops up in the middle of roads is a security barrier, which I think are hydraulic and rise to wholly block even heavy vehicles up-to-and-including heavy trucks.