。
I know those people well.
Literal
I [topic-は] those people [object-を] well know-am.
知っている is the ~ている form of 知る — 'to know' as an ongoing state, since 知る itself is a momentary verb meaning 'to come to know.' This is a major irregularity for English speakers: 知る is technically inceptive ('to learn'), so 'I know' must be 知っている. The negative 知らない skips the ~ている entirely.