In order for Translation Manager to translate your content it needs to know what values in your pages are text values.
Calculating what is or isn't text content is done with value mappers. Value mappers handle the exporting and importing of text values to and from Umbraco property values.
When Translation Manager looks at a content page it will get a list of the property editors the page contains.
For each property editor Translation Manager will get the "Property Editor alias" name for the property (for example Umbraco.Textbox) and see if a value mapper has been loaded that can handle the property.
For some properties the text values are stored directly as the value of a property. but for more complex properties such as properties for The Grid or Nested Content the values are actually stored within a JSON object. The value mappers for these properties are responsible for finding the correct location to read and write the content. Then the core Translation Manager code can concentrate on managing the translation process.
When a translation is created, the value mapper will export the text values out of the Umbraco properties, and Translation Manager will store the text value in a translation node. These text values can then be used by a translation connector to create the translation files.
When you approve a translation job, the content is taken from the translation node and put back into Umbraco. Then the process is reversed, and Translation Manager will use the value mapper to take the text values and put them back into the Umbraco properties.