Questions & Answers about Положи решётку на мангал, а я принесу шампуры.
Положи is the informal singular imperative of положить, so it means put! / place!
A few things are happening here:
- положить is a perfective verb
- the imperative положи tells someone to do the action as a single completed act
- it sounds natural when you mean put it down / place it there
In this sentence, the speaker is telling one person casually to do something:
- Положи решётку на мангал = Put the grill grate onto the brazier
If you were speaking:
- to several people, or
- to one person formally,
you would say положите.
Compare:
- клади / кладите = more like be putting / put it down in an imperfective sense
- положи / положите = put it (and have it placed there)
In everyday instructions, положи is very common.