What is NATO?
The North Atlantic Treaty Organization (NATO) is a group of European and North American countries. They’re like a group of friends who have agreed to defend each other if anyone attacks any of them.
NATO was formed in 1949 and its headquarters are in Brussels, Belgium. In addition to defending each other, NATO members conduct disaster relief work, counterterrorism activities, etc. Recently, Finland became NATO's 31st member.
Why did Finland join NATO?
Finland’s move to join NATO was motivated by Russia’s invasion of Ukraine. It feared that it would be unable to defend itself against such an attack. So, it decided to join NATO for additional security.
This move would give Finland access to increased military and diplomatic strength from other NATO members.
How did Russia and Ukraine react to this news?

Russian officials have announced plans to increase military presence in their Western and Northern areas. Russia has warned that if NATO troops or equipment are placed in Finland, it will increase its military presence near the country.
Ukraine welcomed Finland's move. They expect that by adding a new member, NATO's power will grow, which may help stop the Russia-Ukraine war.