This is the 15995th most frequent Russian word.
прямых
"Прямых" means "of straight" or "direct" (genitive plural or accusative plural form).
Here, 'прямых' is used as an adjective to describe 'линий' (lines), meaning 'straight'.
Я изучаю свойства прямых линий.
I study the properties of straight lines.
'Прямых' acts as an adjective here, describing 'людей' (people), meaning 'straightforward' or honest individuals.
Познакомимся с прямых людей на встрече.
We will meet straightforward people at the gathering.
In this context, 'прямых' modifies 'углов' (angles), denoting 'right angles' as in geometry.