1. To order or cause someone to enter or become involved in something. A noun or pronoun can be used between "send" and "in." Don't worry, I'm sending in our top legal adviser to handle the details. We've sent police in to keep the peace during the protest.

2. To submit or dispatch something, typically a document or payment. A noun or pronoun can be used between "send" and "in." I've sent my application in, so hopefully I'll hear back soon. The employees have sent in a letter of complaint to upper management about their working conditions. Did you send in the check for our utility bill yet?