Specifically, it adds an address into the wallet.

Easy to test now thanks to https://github.com/bitcoin/bitcoin/pull/14410
```
diff --git a/test/functional/wallet_importmulti.py b/test/functional/wallet_importmulti.py
index 9ba6860..bff37b2 100755
--- a/test/functional/wallet_importmulti.py
+++ b/test/functional/wallet_importmulti.py
@@ -95,6 +95,7 @@ class ImportMultiTest(BitcoinTestFramework):
         assert_equal(address_assert['iswatchonly'], True)
         assert_equal(address_assert['ismine'], False)
         assert_equal(address_assert['timestamp'], timestamp)
+        assert_equal(address_assert['ischange'], True)
 
         # ScriptPubKey + internal + label
         self.log.info("Should not allow a label to be specified when internal is true")
```

{
    int64_t scanned_time = wallet.RescanFromTime(time_begin, reserver, update);
    if (wallet.IsAbortingRescan()) {
        throw JSONRPCError(RPC_MISC_ERROR, "Rescan aborted by user.");
    } else if (scanned_time > time_begin) {
        throw JSONRPCError(RPC_WALLET_ERROR, "Rescan was unable to fully rescan the blockchain. Some transactions may be missing.");
    }
}
UniValue importprivkey(const JSONRPCRequest& request)
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 3)
        throw std::runtime_error(
            "importprivkey \"privkey\" ( \"label\" ) ( rescan )\n"
            "\nAdds a private key (as returned by dumpprivkey) to your wallet. Requires a new wallet backup.\n"
            "Hint: use importmulti to import more than one private key.\n"
            "\nArguments:\n"
            "1. \"privkey\"          (string, required) The private key (see dumpprivkey)\n"
            "2. \"label\"            (string, optional, default=\"\") An optional label\n"
            "3. rescan               (boolean, optional, default=true) Rescan the wallet for transactions\n"
            "\nNote: This call can take over an hour to complete if rescan is true, during that time, other rpc calls\n"