If the app icons show an iCloud icon under them next to their name, it means they have been offloaded.
Tapping on them once should cause them to install. The name will change to "Loading..." or "Installing..." and a progress circle should appear on the icon while it downloads.
If you tap on them again while they are installing it will pause the installation.


Once re-installation is completed, the iCloud icon should disappear and the icon on the home screen will again be in full color.
Tapping on it then should open it.
If not connected to Wi-Fi they may not complete the download however.