Fields are used to define UI elements on a block. These include string labels, images, inputs for literal data such as strings and numbers, and rich editors like date pickers and angle pickers.
Blockly provides several fields by default to handle a variety of user input. These can be customized through field specific configuration or with validators, which tell a field how to handle specific content in a standard editor.
→ More info on Built-in fields.
→ More info on Validators.
If you need a fully custom field it takes a bit more work and you'll want to read most of the fields documentation. Start by learning about the parts that make up a field or the overview of creating custom fields.
→ More info on the anatomy of a field.
→ More info on creating custom fields.