what is callback function?

1. could you please explain code (fastseries,cb)
2. how to use fastseries
3. I dont understatnd cb ,
   what is promisify

const { readFile } = require('fs')
const series = require('fastseries')()
const files = Array.from(Array(3)).fill(__filename)

const print = (err, data) => {
  if (err) {
    console.error(err)
    return
  }
  console.log(Buffer.concat(data).toString())
}

 const readers = files.map((file) => {
  return (_, cb) => {
    readFile(file, (err, contents) => {
      if (err) cb(err)
      else cb(null, contents)
    })
  }
})

series(null, readers, null, print)