What is the single most important thing to protect?

Your seed phrase (also called a recovery phrase). It is the list of words your wallet generates when you set it up, and it is the master key to your Malairte. Anyone who gets it can take your coins, and if you lose it nobody can restore your funds. Write it on paper, keep it offline in a safe place, never photograph it or store it in the cloud, and never enter it into any website. Protect the seed phrase and you protect everything.