Captcha stands for Completely Automated Public Turing Computers and Humans Apart (= a mechanism that differentiates between humans and computers).
It is used to ensure that certain functions are not used by a script, but by a human.
On Kaufmich, for example, the captcha is used when writing date reviews or creating a ticket.
The functionality is very simple:
first a small window appears that says "I'm not a robot"
click the checkbox
then click on the appropriate images for the above mentioned term
the pictures change if all matching ones are clicked
click on the matching pictures again
until the captcha disappears
If, for example, several reports are written one after the other, the captcha usually only needs to be edited once.