Let's say you have a GitHub Action workflow with two jobs, foo-job and bar-job. By default, both jobs will run simultaneously.

name: my-workflow
run-name: ${{ github.workflow }} run by ${{ github.actor }}
on:
  workflow_dispatch:
jobs:
  foo-job:
    runs-on: [linux]
    steps:
      - name: echo Hello
        run: echo Hello
        
  bar-job:
    runs-on: [linux]
    steps:
      - name: echo World
        run: echo World

needs can be used so that foo-job must be completed before bar-job will be started.

name: my-workflow
run-name: ${{ github.workflow }} run by ${{ github.actor }}
on:
  workflow_dispatch:
jobs:
  foo-job:
    runs-on: [linux]
    steps:
      - name: echo Hello
        run: echo Hello
        
  bar-job:
    runs-on: [linux]
    needs: foo-job
    steps:
      - name: echo World
        run: echo World