Adam Clarke on Genesis 18:4

Let a little water - be fetched, and wash your feet, etc. - In these verses we find a delightful picture of primitive hospitality. In those ancient times shoes such as ours were not in use; and the foot was protected only by sandals or soles, which fastened round the foot with straps. It was therefore a great refreshment in so hot a country to get the feet washed at the end of a day's journey; and this is the first thing that Abraham proposes.

Rest yourselves under the tree - We have already heard of the oak grove of Mamre, Gen 12:6, and this was the second requisite for the refreshment of a weary traveler, viz., rest in the shade.