I know square brackets mean optional components, vertical bar means OR, but what do curly brackets {} mean?

An example:
"
rangestat slist [if] [in] , interval(keyvar low high) [ options ]

where slist is composed of one or more of the following

(stat) { varlist | new_varname=varname } [ { varlist | new_varname=varname } ...]

(flex_stat) varlist

"