Translation:There is a tall fence around that garden.
"az" and "a" are the same word that just appears in different forms (as in, say, French, where "le" must be "l'" in front of a vowel). So there is an "az" here, it's just spelled and pronounced as "a" ;-)
If you want to take it step by step, you might think of it something like taking "az a kert" ("that garden"), inserting "körul" at the required positions, giving "az körül a kert körül", and then changing the first "az" into an "a" because it's in front of a consonant. But I'm not sure that helps.