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?

It's "those"

"Тех" is a Genitive or Accusative or Prepositional case of the demonstrative plural pronoun "те" - "those". Mostly it is used in the phrase: "с тех пор". It means "since then". But also it is a prefix. As a prefix it means "technic, technical", for example: техосмотр - official checkup / examination / inspection OR warrant of fitness