The ability is often found primarily in the Water Civilization, usually unconditionally. However, it can also be found secondarily in the Nature Civilization based on power. For example; Huckle Kirin Sawyer, Jungle Governor allows your creatures and itself to be unblockable by creatures that have less power than them while it's Shield Force ability is active. It is also very rarely seen in the Light Civilization.
When attacking, you tap your creature in order to attack either your opponent directly (or one of their shields) or one of their tapped creatures. However, if your opponent has a creature that has the Blockerkeyword, he can choose to block and redirect your attack to that creature. This often leads to your creature being destroyed, and your original attack choice being left unharmed.
A creature that has the ability to not be blocked by creatures can attack past and ignore any of those creatures with blocker. However, cards that prevent any attacks such as Nariel, the Oracle, Noble Enforcer and Shangri-La, Climax of "Cruelty" can still prevent any of your attacks from occurring. It also does not stop Guardman from preventing the unblockable creature from attacking what creature it wants to attack.
While a general creature with the ability still suffers from summoning sickness and generally a low power, evolution creatures that can't be blocked (such as Crystal Lancer or Pacific Champion) can be quite powerful for surprise and return attacks.
The Stealthkeyword is very similar to this, providing "this can't be blocked" to creatures if your opponent has a specific civilization in their mana zone.
/ Creature 
Blocker keyword, he can choose to block and redirect your attack to that creature. This often leads to your creature being destroyed, and your original attack choice being left unharmed.