A slot comprises the operation issue and data paths machinery surrounding a collection of one or more functional units (FUs) which share these resources. The term slot is common for this purpose in the VLIW world where the relationship between operation in an instruction and pipeline to execute it is explicit. In dynamically scheduled machines the concept is more commonly called an execute pipeline.
Modern conventional CPUs have several compute pipelines (say two ALU, one FPU, one SSE/MMX, one branch) each of which can issue one instruction per basic cycle but can have several in flight. These are what correspond to slots. The pipelines may have several FUs - an adder and a multiplier, say - but only one FU in a pipeline can be issued to in a particular cycle. The FU population of a pipeline/slot is a design option in a CPU.