Blink allows you to highlight a message to check out later. When a message is highlighted it will get a yellow background in the chat window and it will appear in the Highlights tab in your Inbox.
People use this in lots of different ways. It can act as a todo-list, a quick reminder or simply as a place to store a message you know you're going to use later.
Here's how it works
Simply hover over (or long-press on mobile) the message you want to highlight and the click Highlight on the menu that appears.
Below you can see a message being highlighted in a channel for later use. In this case an address that will be useful to have to hand later.
Note that the highlighted message is available easily even when the user is in a another chat later on. Messages highlighted on the desktop are instantly available on mobile too.