It's just a pronoun that doesn't refer to anything in particular. Knivene hasn't been mentioned earlier, therefore you use the neutral det (used for singular and plural and all grammatical genders).
"Du kan ikke stjele knivene siden de er våre kokkers kniver." = "You can't steal the knives since they are our cooks' knives."
Here you use a pronoun that agrees with the noun it replaces, since it has been mentioned already.
This and these are equivalent to denne(m, f)/dette(n) and disse(pl).
EDIT: Simply put: Det is used to introduce nouns, whether they are mascunline, feminine or neuter, and whether they are singular or plural.