Here armato is being used as an adjective meaning armed.
It comes from the verb armare (to arm), so it is historically a past participle, but in this sentence you can think of it simply as an adjective describing the police officer's condition.
So:
Because poliziotto is masculine singular, and the adjective must agree with it.
Italian adjectives often change form to match gender and number:
Since il poliziotto is masculine singular, armato is the correct form.
A simple English-friendly approximation is:
eel po-lee-TSEE-OT-to eh ar-MA-to
A few useful pronunciation points:
If you want to sound more natural, make the double tt in poliziotto a little longer than a single t.