If personal information such as a login password travels unencrypted over the Internet, it can very easily be intercepted by an eavesdropper. If you are logging into any website, you should make sure that the site offers HTTPS encryption, which protects against this kind of eavesdropping. You can verify this in the URL bar: if your connection is encrypted, the address will begin with “https://”, rather than “http://”.
The following visualization shows what information is visible to eavesdroppers with and without Tor Browser and HTTPS encryption: