It means we're not confident the API is stable yet. There might be further changes before the final, non-experimental version, depending on user feedback and further experience with the current proposal.
In the case of encoding/json/v2, enabling GOEXPERIMENT=jsonv2 has two major effects:
1. It flips encoding/json (the original, not /v2) to use the new implementation. This is supposed to be a fully backwards-compatible change, modulo some changes to the text of some errors. We're very interested to hear of any cases of existing programs breaking when the experiment is turned on, because (aside from the aforementioned error text, which you shouldn't be depending on) it likely indicates a bug that needs fixing. This is the "new code, please test" half of the change.
2. It enables the new API (encoding/json/v2, encoding/json/jsontext, some new options in encoding/json). This is the "unstable API, might change in response to feedback" half of the change.
In the case of encoding/json/v2, enabling GOEXPERIMENT=jsonv2 has two major effects:
1. It flips encoding/json (the original, not /v2) to use the new implementation. This is supposed to be a fully backwards-compatible change, modulo some changes to the text of some errors. We're very interested to hear of any cases of existing programs breaking when the experiment is turned on, because (aside from the aforementioned error text, which you shouldn't be depending on) it likely indicates a bug that needs fixing. This is the "new code, please test" half of the change.
2. It enables the new API (encoding/json/v2, encoding/json/jsontext, some new options in encoding/json). This is the "unstable API, might change in response to feedback" half of the change.