A root word is the main body of a word either in its own right or before affixes (prefixes and suffixes) are added.
This can often help with spelling. How many students spell ‘always’ with two Ls? If they knew the root word was ‘way’, the prefix ‘al’ (which doesn’t have two Ls) and the suffix ‘s’, a student would be less likely to make this spelling error. A similar idea with hope/ful.
There is also a root which isn’t its own word. Reject for instance is the prefix ‘re’ and Latin root ‘ject’.
Here are some links: