can anyone please explain Laravel and react js bootstrap code not able to understand

Following is the code include in bootstrap of laravel code bu the m unable to understand . can anyone please exlain me the code. Thanks

window._ = require('lodash');//whats "window._"  and lodash?
 try {
window.$ = window.jQuery = require('jquery');
require('bootstrap-sass');//wher is this ans its use?
 } catch (e) {}
window.axios = require('axios'); whats "window.axios"  and where is axios?
window.axios.defaults.headers.common['X-Requested-With'] = 'XMLHttpRequest';

whats " window.axios.defaults.headers.common"?

via Chebli Mohamed