SOURCE

const panels = document.querySelectorAll('.panel')

panels.forEach(panel => {
    panel.addEventListener('click', () => {
        removeActiveClasses()
        panel.classList.add('active')
    })
})

function removeActiveClasses() {
    panels.forEach(panel => {
        panel.classList.remove('active')
    })
}

 <div class="container">
      <div class="panel active" style="background-image: url('https://images.unsplash.com/photo-1558979158-65a1eaa08691?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=1350&q=80')">
        <h3>Explore The World</h3>
      </div>
      <div class="panel" style="background-image: url('https://images.unsplash.com/photo-1572276596237-5db2c3e16c5d?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=1350&q=80')">
        <h3>Wild Forest</h3>
      </div>
      <div class="panel" style="background-image: url('https://images.unsplash.com/photo-1507525428034-b723cf961d3e?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=1353&q=80')">
        <h3>Sunny Beach</h3>
      </div>
      <div class="panel" style="background-image: url('https://images.unsplash.com/photo-1551009175-8a68da93d5f9?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=1351&q=80')">
        <h3>City on Winter</h3>
      </div>
      <div class="panel" style="background-image: url('https://images.unsplash.com/photo-1549880338-65ddcdfd017b?ixlib=rb-1.2.1&ixid=eyJhcHBfaWQiOjEyMDd9&auto=format&fit=crop&w=1350&q=80')">
        <h3>Mountains - Clouds</h3>
      </div>

    </div>

/* @import url('https://fonts.googleapis.com/css?family=Muli&display=swap'); */

.container {
  display: flex;
  width: 90vw;
}

.panel {
  background-size: cover;
  background-position: center;
  background-repeat: no-repeat;
  height: 80vh;
  border-radius: 50px;
  color: #fff;
  cursor: pointer;
  flex: 0.5;
  margin: 10px;
  position: relative;
  -webkit-transition: all 700ms ease-in;
}

.panel h3 {
  font-size: 24px;
  position: absolute;
  bottom: 20px;
  left: 20px;
  margin: 0;
  opacity: 0;
}

.panel.active {
  flex: 5;
}

.panel.active h3 {
  opacity: 1;
  transition: opacity 0.3s ease-in 0.4s;
}

/* @media (max-width: 480px) {
  .container {
    width: 100vw;
  }

  .panel:nth-of-type(4),
  .panel:nth-of-type(5) {
    display: none;
  }
} */