What do these marks mean?

(This is in 4/4) Just to double check this rhythm--I believe they're eight note triplets (as per the first measure), marked "6" as in 6 beats.

Can anyone confirm this or explain how I would read this? I thought a dash through the note stem would indicate that they're quarter notes (like half notes double time are quarter notes), but maybe they're eight notes? I'm kind of just confused how this works, if anyone could explain I'd appreciate it!