The nextTick method is just a comfortable way to execute a function after the data has been set, and the DOM has been updated. As an example, the usage is going to be similar to setTimeout:
// modify data
vm.msg = 'Welcome to Vue'
// DOM not updated yet
Vue.nextTick(function () {
// DOM updated
})
// usage as a promise (2.1.0+)
Vue.nextTick()
.then(function () {
// DOM updated
})