Explain the concept of a pipeline "stall."?

It's when the a "bubble" is introduced into the pipeline. More precisely, it's when there's a period where a pipeline stage isn't active. The bubble also occurs in subsequent stages after where the bubble first occurs. This can happen when there are data or structural hazards. A data hazard may arise when an assembly instruction expects a value in a register that isn't there yet. The pipeline must delay all instruction processing until the variable is ready or the computation will be wrong. A structural hazard may arise when two stages try to use the same computation resources. For example, the pipeline was designed to let the instruction decode stage and the execution stage use the ALU.