What is the correct way to use gulp-bem?

Question

BEM

There is an example on github:

var gulp    = require('gulp');
var bem     = require('gulp-bem');
var concat  = require('gulp-concat');

var levels = ['base', 'blocks'];
var tree = bem(levels);

tree.deps('blocks/page')
    .pipe(bem.src('{bem}.css'))
    .pipe(concat('index.css'))
    .pipe(gulp.dest('./dist'));

It is not clear to me what kind of path is specified here tree.deps('blocks/page') and what it is for. Tell me please. Thank you!