They are demostrative pronouns, similar to "that".
BTW the English definite article "the" evolved centuries ago from a demostrative pronoun at about the time when English lost cases. It's a common ocurrence. Same thing happened in other Germanic languages, in Romance languages (Latin didn't have articles, but it had cases), and two Slavic languages that also lost cases.
Since Czech still very much uses cases (declination), it doesn't need articles. Sometimes, Czech uses a demonstrative pronoun like "ten" where English would use "the", but it's often optional and is not demanded by grammar the way it is in English.
For example "Ta strana ví všechno" or "Ta vláda ví všechno" is only used in a specific, jovial context (similar to "That government - it knows everything!". The natural and neutral way to say these is without the demonstrative.