If someone was to steal my external hard drive would they need to input my password to be able to read the TimeMachine backup data on there?
Not by default.
By default Time Machine Backups are not encrypted. They are stored in a sparse bundle disk image with the name of the computer which the backup is from. You can open this image and browse it without a password.
You can easily enable encryption for Time Machine backups if they are connected via USB/Firewire. It's more complicated if you access your backup disk using ethernet/Wi-Fi. How can I enable encryption for my Time Machine backup?