It’s not uncommon to stumble across a word with which you’re unfamiliar, especially when reading a document written by someone else.
Google Docs includes a helpful tool to let you look up a word’s definition from right within the document. With your cursor inside the word in question, just use Tools > Define (or Ctrl + Shift + Y):
A sidebar will pop open with the word’s definition:
Docs will do its best to interpret the word which is at the root of your request. For instance, in the example above I’ve used a plural, cobbles, but Docs has correctly inferred that I want a definition for cobble. It’ll do a similar thing with, for example, comparitives and superlatives of adjectives: