Learn Hardcore Serbian: Tvoj kaput je ispod stolice, a moj je iznad torbe na polici. - Your coat is under the chair, and mine is above the bag on the shelf.

Because Serbian often leaves out a noun if it is already obvious from context. In the second clause, moj means my one / mine, with kaput understood.

So:

Tvoj kaput = your coat
moj = my coat / mine

Grammatically, moj is still matching the omitted noun kaput, so it stays masculine singular nominative. If the omitted noun were feminine, the form would change:

Moja je ispod stola. = Mine is under the table (with a feminine noun understood, like torba)

Why is je repeated, and why does it come after moj?

There are really two clauses here:

Tvoj kaput je ispod stolice
a moj je iznad torbe na polici

Each clause has its own je (is), so repeating it is normal.

Also, je is a clitic in Serbian. Clitics usually go in the second position of their clause. That is why you get:

moj je
not je moj

You may sometimes hear the second je left out in casual speech if the meaning is clear, but the full version with je is standard and very natural.