Estonia considers itself a front-line state, a Nato member where its border guards stare across the Narva River at the Russian fortress of Ivangorod.
This tiny Baltic state, once a part of the Soviet Union, is convinced that once the fighting stops in Ukraine, President Vladimir Putin will turn his attention to the Baltics, looking to bring countries like Estonia back under Moscow’s control.
To help stave off that possibility, Estonia’s government has poured money and weapons into Ukraine’s war effort, donating more than 1% of its GDP to Kyiv.
Here, you can go read the Wikipedia about NATO. If you look at the section about the various articles, you will find your questions answered, and see the myriad other ways NATO works and enables mutual assistance.
https://en.m.wikipedia.org/wiki/North_Atlantic_Treaty