When you head to Settings > Regional settings, you are asked to select a time format, currency and day the week starts on.
But what do they mean?
The time format you select is used in places like the account timelines and task due dates.
Week starts on
When you select a day for the week to start on, this is updated for you in the date pickers across the app.
Your currency is used for setting targets, opportunity values and the insights dashboard. This setting is for the entire team.