what does 'use strick' mean in gulp?

'use strick';

var gulp = require('gulp'),
    concat = require('gulp-concat'),
    uglify = require('gulp-uglify'),
    rename = require('gulp-rename');

gulp.task("concatScripts", function(){
    //combine all these scripts
    gulp.src([
        'js/jquery.js', 
        'js/sticky/jquery.stick.js', 
        'js/main.js'])
    //create file
    .pipe(concat("app.js"))
    // put in folder destination
    .pipe(gulp.dest("js"));

});

gulp.task("minify", function(){
    gulp.src("js/app.js")
        .pip(uglify())
        .pip(rename("app.min.js"))
        .pipe(gulp.dest("js"));

});

gulp.task("default", ["hello"], function(){
    console.log("this is the default task");
});