This command contrasts the contents of the files in your current working directory with those found in the last commit (referred to as HEAD).
It effectively gives you a summary of all the changes that you've made but not yet committed to the repository.
It includes information such as additions (marked with +) and subtractions (recognized by -). For comparing with other commits or branches, simply replace HEAD with the desired commit hash or branch name. For example, git diff HEAD~1 compares with one commit before the last.
Moreover, you can use --stat to see a summary instead of the full diff, or --color-words to see changes at the word level, instead of by line.