What is variable.on meaning with meteor, mongodb and reactjs?

Question

I’m trying to understand what this function is doing and I have a question regarding the below code.

const saveFile = (event, files) => {

    const file = new FS.File(f);
    file.profileId = card.profileId;
    file.cardId = card.cardId;

    const upload = UserFiles.insert(
      {
        file: f,
        streams: 'dynamic',
        chunkSize: 'dynamic',
        meta: {
          profileId: card.profileId,
          cardId: card._id,
        },
      },
      false,
    );

    upload.on('end', function (error, fileObj) {  // here is my question
      if (error) {
        alert(`${error}`);
      } else {
        console.log(fileObj);
      }
    });
    upload.start();
};

Also, I’m using the package called Meteor-CollectionFS and GridFS.

My question is what upload.on('end', function (error, fileObj) {...... means? What is this part of the code doing?

Also

    const upload = UserFiles.insert(
      {
        file: f,
        streams: 'dynamic', .....

is a little strange to me.

I understood they were trying to insert values to UserFiles collection , but why are they const upload for this inserting?