Thankfully means that something has happened or will happen as a result of being thankful. It's like a good outcome comes from being grateful.
When we say "thankfully," it usually means that things turned out better than expected, often because someone was kind or helpful. For example, if you're late for work and the traffic is terrible, but then you see a parking spot right in front of your office building, you might say "thankfully" about finding a space to park. You're relieved it all worked out okay. If you have a bad cold and someone makes you soup, you might say "thankfully, it tastes so good." In this case, you're happy that the person was kind enough to cook for you, even though you don't feel well.
The word "thankfully" can also be used to talk about being grateful for something that's happening right now. For instance, if it's really cold outside but your apartment is warm and cozy, you might say "I'm so thankful I have a good heating system." In this case, you're happy about the situation as it stands.
It's worth noting that "thankfully" can be used in both written and spoken English. If you hear someone say it out loud, they probably mean that things turned out well or that they're happy for some reason.
In a thankful manner; giving thanks.
