When there's an uncomitted change on a branch, is that readable by anyone reading that branch? And so a commit is just a way of delineating and referencing particular changes to a branch, rather than anything to do with making visible the change, is that right?

That's correct. Additionally, merging from

source

to

destination

will only merge the latest committed data on

source

.

This is also where protected branches come in handy - they stop you from writing directly to a branch, effectively not allowing uncommitted changes.

👍