"mirad!" is one example of the differences between Spanish in Spain and the way it is spoken in Latin America. it is the (2nd person) plural imperative (command) of the verb mirar in Castilian Spanish, as spoken by the Spaniards. Its equivalent in Latin American Spanish is "miren!" As Xuorarch points out, it means "look, you guys" (i.e. several of you, look).
To complicate things further, In Latin America, the imperative comes in 2 flavors:
mira! (stress on the 1st syllable): look! (singular) corresponds to "tú miras", spoken by most Latins.
In Argentina, Uruguay (partly) and parts of Central America, the second person singular "tú" is not used; here they use a form that is no longer used in Spain "vos" in which the second person singular is conjugated thus: vos mirás (stress on 2nd syllable). And the corresponding imperative form is "mirá" (stress on the second syllable).
(However, this vos form corresponds in the plural to the Spanish vosotros.)
In graphic form:
tú (singular) -- Ustedes (plural) -- mira! (sing.) -- miren! (pl) -- most of Latin America.
vos (singular) -- Ustedes (plural) -- mirá (sing.) -- miren! (pl) -- Argentina, Central America (Uruguay is a special case because they mix "tú" with the "mirás" form).
tú (singular) -- vosotros (plural) -- mira! (sing) -- mirad! (pl) -- Spain
Hope this clarifies things.