Hi Ariel,
Distributable licenses refers to a type of license deployment where a customer installs their own license server an this allows individual users to activate a permanent license on their own computer from a pool of licenses stored on that server.
Concurrent licenses refers to a type of license that is temporarily checked out from a license pool and automatically returned after use. This license type also requires an on-site license server.
Concurrent licenses are useful when you have a larger group of infrequent users. For example, say you had 50 users but usually less than 10 are running MapInfo at any given time. In that case a pool of 10 concurrent licenses might serve their needs, instead of 50 permanent licenses.