Cronbach's alpha (or tau-equivalent reliability) is a measure of the relationship between a group of questions. The group of questions is called a scale and each question in that group is an item. Cronbach's alpha is therefore a measure of the internal consistency of a scale and thus of the strength of its reliability.
More details about Cronbach's alpha
https://datatab.net/tutorial/cronbachs-alpha
And the Cronbach's alpha calculator
https://datatab.net/statistics-calculator/reliability-analysis/cronbachs-alpha-calculator