If it is connected directly to the term/duration of tenancy, then 2+1 would mean that you are signing on for a 2 year duration (ie neither party can terminate the tenancy in the first two years without breaching the contract), and the +1 would be an automatic renewal of the 3rd year.

You are however allowed to terminate the contract from automatically renewing for the third year without any breach/penalty. The terms should either party breach the contract and the duration for termination before the 3rd year renews should be outlined in the agreement as well.