The Split Text column splits up text by a value of your choosing. For example, say you have a list of people separated by commas.
Marie Rao, Tamsin Woodward, Alan Eijk, Fo Lambert, Christina Wallace, Ryan Holbert
Because this value lives in a single cell, Glide sees it as a single value. However, you can split it up by the commas.

When you do this, you end up with a new column with multiple values.

The image below has created a Relation column with the Joined List column that relates all employees who are attending this meeting.

Because Split Text is splitting up multiple distinct pieces of information, the column you apply this to should have multiple values in each row—otherwise known as a string.
Adding and Configuring a Split Text Column
In the Data Editor, open the table you want to edit.
Add a new column by clicking the plus (+) symbol to the right of the table.
New columns added with the plus button will always appear on the right. If you'd like to add a column somewhere else in your data, you can select the dropdown menu on an existing column and choose Add column right. You can also click and drag columns to rearrange them.
Give the column a name.
For Type, search for Split Text or navigate to Computed → Split Text.
Choose the column that contains the text you want to split, and then type in the item in that column to act as the separator. This could be one of many possible options, including a comma, period, space, hyphen, etc. depending on the value in the column.