What does it mean the letter X in Zoho Analytics

What does it mean the letter X in Zoho Analytics


I am translating a document  about the use of  dashboards in  Zoho Analytics ,
 and I found these sentences :

Tap any where to select corresponding X entries

Tap on X-axis label to select all entries for the X

Swipe down to filter all data points for any X

Swipe down anywhere to filter bubbles of entire X

What does it mean  the letter  X  in this context ?