Umami is now generally considered to be the fifth taste.
The usual four tastes which we all learnt at school are sweet
Umami tastes are often found in meaty or savoury dishes. Think of a meaty broth, or fresh ripe tomatoes, mushrooms or shellfish. These are all foods which have been described as having umami.
Umami is derived from the Japanese words umai-"delicious" and mi-"taste". Umami is generally considered to mean "pleasant savoury taste."
In the scientific world research has shown that this taste relates to the taste of glutamate and nucleotides found in our food. There is a great article in Wikipedia if you want to go into it more.
Suffice it to say- Umami's basic effect is to balance the taste and round out the total flavour of a dish.
Like many things, when you combine several umami tastes together the whole taste sensation is greater than the sum of the parts. For instance the Japanese dashi with kombu seaweed and dried bonito flakes, the Scottish dish of cock-a-leekie soup and the Italians add Parmesan cheese to tomato and mushroom sauces.
As for black garlic, try slicing it onto tomatoes on crackers with a little brie.