The “choose one or more tags” filter allows you to select a tag, in order to find all tasks on which the chosen tag has been allocated. There is no limit on the number of labels that can be selected.
Please note that if you select more than one tag, you will see the option "AND" or "OR" appear next to it.
If you leave the default choice "AND", you will find the tasks with all the selected tags (e.g. as in the screenshot below, you will see all tasks with the tags "machine critique" and "preventif" simultaneously).
If you select the "OR" option, you will find all the results matching at least one of the selected tags (e.g. you will find the tasks that have the "machine critique" tag, but that have not the "preventif" tag, and vice versa).