When enabled, the Variables Assumed Non-Negative checkbox on the General Solver tab:

tells LINGO to place a default lower bound of 0 on all variables.  In other words, unless otherwise specified, variables will not be allowed to go negative.  Should you want a variable to take on a negative value, you may always override the default lower bound of 0 using the @BND() function.  If this option is disabled, then LINGO’s default assumption is that variables are unconstrained and may take on any value, positive or negative.  Unconstrained variables are also referred to as be being free.

By default, LINGO enables the non-negative option, thereby setting a default lower bound of 0 on all variables.