Content Browser
The unreal content browser allows you to explore the assets (textures, materials, meshes etc.) that you can place into your level. There are many available and you should look through the packages to see what fits best with your world. When you first start the content browser, you will see a panel on the left that lets you explore different collections and packages. On the right , you can filter to search for the item you want.
When unreal content browser first starts, it may not have loaded the contents of the packages into memory and thus, you may see only a few of the assets in a particular package. To ensure that you are seeing all the content, right click on the package in question and choose fully load.
What you will see for each of the various types of contents is a preview snapshot of the object. The type of object is labeled in each of the previews but it can also be identified by the colour of the border. To explore each type of content further you can double click and set different properties.
Context Menu
Once you select an asset in the content browser, placing it into your level requires the use of the context menu. With the asset selected, if you right click into one of your view ports, it will bring up the context menu.
The context menu will typically have an entry based on your selected asset. For example if you have selected a material in the content browser, then right clicking on a surface will bring up the "apply material" menu item. This menu item will apply or place the selected asset to your map.