Lactones look like this:
Cyclic: the atoms form a ring
"Ester" refers to the oxygen double-bonded to the carbon, and the other single-bonded oxygen next to it.
Ester:
The "R"s in the picture refer to any combination of carbon and hydrogen atoms.
"1-oxacycloalkan-2-one structure" - The single bonded oxygen is at position 1 ("1-oxa"). There is a ring ("cyclo") of some combination of carbon and hydrogen atoms ("alkan"), next to an oxygen double bonded to a carbon ("2-one" -- note "one" is not the number 1, but is a suffix indicating a double bonded oxygen, as in "ketone" or "acetone"--or "lactone" for that matter)
So here is kavain, a kavalactone. You can see it is a cyclic ester, with a bunch of stuff hanging off (or your could say the "ring" is complicated) characteristic of a
kava-lactone: