bouncers are standalone software pieces in charge of acting upon a decision taken by crowdsec : block an IP, present a captcha, enforce MFA on a given user, etc.
They can either be within the applicative stack, or work out of band :
nginx bouncer will check every unknown IP against the local API before letting go through or serving a 403 to the user, while a firewall bouncer or a cloudflare bouncer will simply "add" malevolent IPs to nftables/ipset set of blacklisted IPs.
Bouncers rely on crowdsec's Local API to be able to get information about a given IP or such.
You can explore available bouncers on the hub.
To be able for your bouncers to communicate with the local API, you have to generate an API token with
cscli and put it in your bouncer configuration file:
sudo cscli bouncers add testBouncer
Api key for 'testBouncer':
Please keep this key since you will not be able to retrieve it!
This command must be run on the server where the local API is installed (or at least with a cscli that has valid credentials to communicate with the database used by the API). This is only necessary if you "manually" install a bouncer, packages and install scripts usually take care of this.
If you were to create your own bouncers, look at this section of the local API documentation.