Understanding BinOpr

I can't figure out what  "enterlevel" function is doing here. I am  aware that the below has implicit stack. I am also aware that "nCcalls" is the number of  of nested calls.

https://www.lua.org/source/5.2/lparser.c.html

static BinOpr subexpr (LexState *ls, expdesc *v, int limit) {

 BinOpr op;
 UnOpr uop;
 enterlevel(ls);
 uop = getunopr(ls->t.token);
 if (uop != OPR_NOUNOPR) {
 int line = ls->linenumber;
 luaX_next(ls);
 subexpr(ls, v, UNARY_PRIORITY);
 luaK_prefix(ls->fs, uop, v, line);
 }
 else simpleexp(ls, v);
 /* expand while operators have priorities higher than `limit' */
 op = getbinopr(ls->t.token);
 while (op != OPR_NOBINOPR && priority[op].left > limit) {
 expdesc v2;
 BinOpr nextop;
 int line = ls->linenumber;
 luaX_next(ls);
 luaK_infix(ls->fs, op, v);
 /* read sub-_expression_ with higher priority */
 nextop = subexpr(ls, &v2, priority[op].right);
 luaK_posfix(ls->fs, op, v, &v2, line);
 op = nextop;
 }
 leavelevel(ls);
 return op; /* return first untreated operator */
}

Regards, Emeka