There are two easy ways to add a content block to a tab:
Pre-populating content
Create the content block
In the Manage workspace, go to Settings > Content > Content blocks
Click New content block
Under Location, choose Tab info box
Compose your content
If desired, denote whether this content block is visible to all users or only those with specific roles under Role visibility
Click Save
Assign content block to tab
Go to Settings > Entries > Tabs
Click on the tab name
Under Content block, choose the content block you just created
Click Save
Using the form editor
From the Manage workspace, go to Entries and click Edit form
Navigate to the tab that you'd like to add a content block to
Click the pencil icon
Add your content
Click Save
The new content block will be available in the list in the Manage workspace underSettings > Content > Content blocks for future editing or you can simply click the pencil icon again from within the entry form to access the content block and make instant changes.