Use arguments and parameters in url routes

Exemple 1 : Form with Node

in routing.yml

my.node.info:

  path: '/user/contactmail/{node}'

  defaults:

    _form: '\Drupal\myx_meet\Form\ContactUserForm'

    _title: 'Contact'

  requirements:

    _permission: 'access content'

  options:

    parameters:

      node:

        type: entity:node

In ContactUserForm

  public function buildForm(array $form, FormStateInterface $form_state, NodeInterface $node = NULL) {

  // OR AccountInterface for $user

    dpm($node);

//............... Your form

}

To use a 'String' as parameters:

    parameters:

      parametername:

        type: String

Another Example with Options, Defalt values ...

mymodule.content.view:

  path: '/content/{type}/{node}/{id}/{title}/{offset}'

  defaults:

    _controller: '\Drupal\mymodule\Controller\ContentViewer::view'

    _title: 'View'

    offset: '0'

    title: 'untitled'

  requirements:

    _access: 'TRUE'

  options:

    parameters:

      node:

        type: entity:node

      title:

        type: String

      id:

        type: Integer

More details : https://www.drupal.org/node/2310425