ReCaptcha is, just like Captcha a security tester. It looks if you are human... and not a robot password generator...
Some time ago you had to fill in 2 stirngs of symbols. One served as a test, the other was used to digitalize the old US newspapers.. They scanned the papers and where the computer could not fid a match, it was presentend to us as in a ReCaptcha. If there was a significant number of people who said the same.. it was assumed to be correct.. A genius idea from Luis Von Ahn. See his video below..
Now a days the developed an other system. Much easier.. Just say you are not a robot.. But if the algoritme thinks you are not thrustworthy... you have to fill in an extra test. The algoritme is secret..
For ReCaptcha you need a Google site key and secret key. Use the link in the configuration to get there.
You need them. You have to add them to your domain. Subdomeins are automatically active too.
If the Form ID is being asked... use the inspector of your browser and look for the ID.
You can test recaptcha by waiting a long time to fill in the test. You will have to do the extras...