No, were isn't a preposition. It's a verb. Were is the past form of verb to Be and it is used in the past . ex:
They were travelled yesterday.
V. to be has 8 forms
be = the verb in the infinitive
am- is- are= the present form of the verb
was- were= the past form of the verb
being= the continuous form
been= used in the present perfect: has been / have been