The word is literal before it is theological. A tirtha is a ford, the shallow place where a river can be waded, and a Tirthankara is the one who makes it. What is forded is samsara, the stream of birth and death, and the crossing that is built is not a doctrine but a community: the fourfold order of monks, nuns, laymen and laywomen, which Jainism calls the tirtha itself. This is the whole content of the title. A Tirthankara is not the one who knows, because omniscience is shared with every soul that reaches liberation, and it is not the one who founds, because Jainism holds its teaching to be beginningless and merely rediscovered. It is the one who builds the crossing and leaves it standing for others.
The consequence is a form of veneration that is regularly misdescribed. A Jain who bows before the image of a Tirthankara is not petitioning him. A liberated soul is held to be wholly beyond action in the world, incapable of hearing a request and incapable of granting it; the reverence is directed at what the figure achieved and models, and the benefit is understood to arise in the one who reveres. Summaries that call Jainism atheistic and summaries that call Jain temple worship prayer are both reaching for the wrong category, and the difference between them is exactly this.
Twenty-four are counted for the present descending half-cycle of time, from Rishabhanatha to Mahavira. Twenty-four is not a total: Jain cosmology gives each ascending and descending half-cycle its own twenty-four, so the number describes an era rather than the history of the universe. Within the era the sequence is fixed, and Digambara and Svetambara, who disagree about a great deal, do not disagree about it.