In general field tags provide the possibility of adding metadata to the fields in your data model.
There is an automatic field tagging based on the content of the data field, but if you want to control the field types you can use num() and text() function in your load statement to cast each field to the type you want to.
If you want to influence the actual metadata tagging for a field you have the tag() / untag() function.
In general, most unrecognized formatting from a data source will now be treated as a text representation where previously, these formats where interpreted numerically, often with incorrect results. For example in Qlik Sense November 2018 version, data fields (defined as text fields in the data source) containing yearmonth data (yyyymm), is read as a number/integer. In February 2019 and April 2019 versions, those data is read as ascii/text.
For more information about field tags please check: https://help.qlik.com/en-US/sense/April2019/Subsystems/Hub/Content/Sense_Hub/Scripting/field-tags.htm