In conclusion, I would like to thank our guest speaker for her fascinating talk.

in 是介词，表示"在...之内"或"处于某种状态"。 conclusion 是名词，来自动词 conclude（结束、得出结论），指"结论、结尾"。 in conclusion 是一个固定短语，用于引出总结性陈述，相当于"总之""最后"。