koi is an adjective for "strong" or "deep" flavor, usui is the opposite, "weak." So this is probably in regards to the strength of the tea's flavor.
御 is an honorific, pronounced "o" or "go," depending on the word. Doesn't really translate to anything, just makes it polite.
抹 is actually pronounced "matsu" by itself, but the "tsu" is turned into a sokuon (signifying a long consonant) when in a compound like this.
茶 just means tea, "cha"