It's a matter of probability. Let's say it's a windy, but sunny summer's day. It's less likely that it will rain, but it's certainly possible with that brisk wind. But it's unlikely, not at all probable, that it will start snowing. Growing up in Finland, that's not impossible either, though. :D

Hi Furkan, The previous answer given is correct. "less likely" just means there is a low chance it could happen. "Unlikely" means it probably won't happen 🙂