to take the place of some other person and do that person's work satisfactorily. (As if you were wearing the other person's shoes.)
I don't know how we'll be able to do without you. No one can fill your shoes.It'll be difficult to fill Jane's shoes. She did her job very well.
Idiom
fill someone's shoesfill (someone's) shoes
to do what someone else has done as well as they did.
He was a great coach, and it's not going to be easy to get someone to fill his shoes.
fill (one's) shoes
To replace one in some role; to take over for one in some position. You do so much for the company that I don't see how anyone could fill your shoes.
fill someone's shoes
Assume someone's position or duties, especially in a satisfactory way. For example, It'll be hard to find someone to fill her shoes when she retires, or John expects his son to fill his shoes at the store. Also see in someone's shoes.
fill someone's shoes (or boots)
take over someone's function or duties and fulfil them satisfactorily. informal