An arbiter is a lightweight node that doesn’t hold any data. This node is used in voting to break ties.
An even number of nodes is not recommended in a production environment. In your scenario of using two data bearing nodes (a primary and a secondary), should one of those instances stop working, you will no longer be able to write to your database as MongoDB needs a majority (the floor of 50% + 1 nodes from the replica set) to be participating (connectable from the other nodes). This is why @satvant_singh was recommending an arbiter on your application server, as long as that’s not on one of the servers hosting your database instances.