That is the plural genitive form of the demonstrative pronoun «тот» (that) if I am not mistaken... I mean, I am not really sure if that is genitive case but anyway, this is how the word «тех» might be used (the 4th example):
- Do you know this person? – Ты знаешь этого человека?
- Do you know these people? – Ты знаешь этих людей?
- Do you know that person? – Ты знаешь того человека?
- Do you know those people? – Ты знаешь тех людей?