Verilog is one way to do design entry -- the process of telling the FPGA tools what you want the FPGA to do. Just like software might be in C or Python or Java, there are many ways to do design entry. It is often possible to use schematics -- really logic diagrams -- to define an FPGA, but there's good reasons why that doesn't work as well on larger projects that we will talk about in this bootcamp.
However, you will many people use VHDL which is very similar in concept to Verilog. However, where Verilog is somewhat similar to C, VHDL is very similar to Ada. There are other options, as well. There are tools that will convert limited subsets of the C language to FPGA configurations, for example. There's also System Verilog which has a lot of additional features, most of which are aimed at simulation.
There are also lesser-used languages. Some are proprietary and have mostly fallen out of use. Some are newcomers that usually look somewhat like another programming language. SpinalHDL and MyHDL are examples. The purpose of all of these things are the same: to inform the FPGA configuration tool what it is you want.
Obviously, not all tools are going to take all languages. However, if a toolset can handle, for example, Verilog and VHDL, you can probably mix these if you want in one project. That may seem crazy, but it is a great benefit if you have libraries or functions from a third party in one language and you want to use another one.
This is usually possible because there is a fairly standard intermediate format called EDIF. If you buy IP (intellectual property) from a vendor, it will probably come as an EDIF file. That means you won't be able to easily edit it, but you can add into your designs.
The tools we are going to use with the IceStick just do Verilog. But if you use commercial tools, you'll likely have an easy time using at least VHDL or Verilog, if you choose. However, this series of bootcamps is going to focus on Verilog.