The word "container" can mean different things depending on the exact technology under discussion;however, containers generally refer to technology for packaging your application and any required software in a reusable, isolated, single unit. Containers are a very popular and common tool for deploying workloads to the cloud.
Containers are packages for software
The main thing containers are used for is packaging up software. When you deploy an application, you usually need other software to support the application. This software can include whatever programs you need to run your code, any software your application calls on to perform work, an operating system, and plenty of other libraries and programs. Containers are a convenient way to bring all of this software together alongside your application so that when you want to deploy your application all you have to do is deploy the container. You do not have to deploy all of the different pieces of software again and again.
Containers are also great for isolating this supporting software that applications need from each other. If two applicatons on the same system need different versions of a single piece of supporting software, you may not be able to deploy the two applications together on that system. If you put the applications in containers, however, you can safely deploy them side-by-side without any conflicts from their supporting software. Of course, you are running duplicates of software in many cases, but that is usually an acceptable tradeoff for the convenience of using containers to deploy applications.
Popular container technologies
There are a variety of container technologies currently in use on the cloud. The most popular or well-known overall is probably Docker, but Docker is not the only container technology. Other container technologies include but are not limited to Podman, Linux Containers (LXC/LXD), Kaniko, RunC, Clear, and Containerd.
All of these container technologies generally try to solve the problem of how to package software into re-usable, isolated units for easier management and distribution.