Disclaimer: The following explanation of CPU ticks is based more on colloquialism than actual computer science for the sake of simplicity.
So a CPU Tick is (non-scientifically) referring to a cycle your CPU goes through to perform an operation. These are sometimes (non-scientifically) referred to as FLOPS or floating-point operations per second. The more GHz your CPU sports, the more FLOPS it can put out. When you ask your computer to perform arithmetic, it’s going to cost a “CPU tick” to do it. One operation will be practically instantaneous, but many many floating-point operations will slow you down. They say that preemptive optimization is the root of all evil, but you should save any CPU time you can. This is the lowest of the low-hanging fruit. For the most part, you don’t need to worry at all about CPU ticks.
Now where this differs from the world or object tick is that an object (the world is an object) is ticked once per frame rather than based on hertz. When an object is ticked, all of its instructions are sent to the CPU and GPU for their internal ticks to handle. The more instructions you have to send, the more finite CPU ticks it will take and the longer it will take for the next frame to be rendered, thus lowering the frame rate. All of your objects’ tick times are added up to the frame update time. When all of your objects’ instructions have been executed, they will be ticked again. Of course, “Tick” isn’t the scientific name for a frame update. Some engines refer to this frame update cycle as “Update” or “Draw.” Unreal just uses the word “Tick.”