Add callouts explaining branches

episodes/02-getting-started.md

@@ -167,9 +167,22 @@ No commits yet
 nothing to commit (create/copy files and use "git add" to track)
 ```
 
-The output tells us that we are on the main branch (more on this later) and that we have nothing to commit (no
+The output tells us that we are on the main branch and that we have nothing to commit (no
 unsaved changes).
 
+:::::::::::::::::::::::::::::::::::::::::  callout
+
+## Branches and commits
+
+Commits in Git are like snapshots of our project.
+They are ordered into sequences, or "branches", with each commit pointing back to the one before it in the branch.
+Git keeps track of the last commit in each branch with a special label;
+in this lesson we mostly use one branch with the label "main".
+
+![](fig/git-branch.svg){alt='The label ‘main’ points to commit c2; c2 points to its parent c1, which points to c0.'}
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
 ### Two steps: Adding and committing
 
 We will now create and save our first project file. This is a two-step process. First, we **add** any files for which

episodes/05-github-pages.md

@@ -133,6 +133,20 @@ it's more practical to grant everyone access to commit directly instead.
 
 ::::::::::::::::::::::::::::::::::::::::::::::::::
 
+:::::::::::::::::::::::::::::::::::::::::  callout
+
+## Branching and merging
+
+Branches get their name because you can create a new one (say, "patch")
+that splits off from an existing one like "main",
+allowing different sets of changes to be made in parallel.
+Later on, Git can take the changes from "patch" and merge (copy) them into the "main" branch.
+In the last challenge, GitHub did all this for us in the background.
+
+![](fig/git-branches.svg){alt='The label ‘main’ points to commit c5; arrows point from c5 to c3 to c1 to c0. The label ‘patch’ points to commit c4; arrows point from c4 to c2 to c1. A dotted arrow points from c5 to c4.'}
+
+::::::::::::::::::::::::::::::::::::::::::::::::::
+
 :::::::::::::::::::::::::::::::::::::::: challenge
 
 ## Optional challenge: Contributing to a page owned by someone else (slightly more complicated way)