What are lymph nodes?

hi i have a lump on my neck on the left side no bigger than a cm length and width, it is not painfull it is just making me worry because it is their. I have been told it could be cancer by various people and its making me worry, other symptoms i have are tiredness before 10pm at night, mild pains in the middle of my back, could you help put my mind at ease thankyou.