When npm installs native node modules it uses node-gyp to compile code. This is the seam node uses for targeting different operating systems, e.g. OS X, linux, Windows etc.
By default node-gyp compiles using one core and if you have more than one you probably want to utilize that power to speed up compile time. The way node-gyp handles this is by using the JOBS environment variable, which sets the jobs variable here. This piece of code then checks the value of jobs to determine how many cores to use.
Note that if the value of JOBS is max then all cores will be used. So lets try this on leveldown. First lets check that JOBS isn't set yet:
lms@ux301|01:34|~/src/leveldb-repos/leveldown (master) $ echo $JOBS