If the bed was amended with a substantial amount of organic material before planting the soil is sinking as that material decomposes and disappears. With the solution being to replace the lost volume with actual soil consisting mostly of mineral material that will not break down and vanish. Just sand by itself being a mineral material would serve this purpose but too much of it added to a soil makes it sandy = prone to leaching and drying out. However maybe the quantity of sand you are talking about here would not be enough to make the bed sandy. It depends on how much sand is already present in the bed and if adding the few hundred pounds to this existing component would then push the bed over into being a sandy one. With sand affecting the texture of a soil significantly starting when the sand makes up more than half of the total volume of soil.

Thank you, and do you think the existing plants need to be removed before adding things? it almost looks like I could remove the ones in the middle of the plot and after adding the sand and woodchips have the whole bed slope down to the borders which is lined by ferns.

There's no picture so I can't tell. One thing I can tell is that digging in the wood chips would continue the assumed existing problem of shrinkage due to bulk organic matter having been added to the soil previously. (And there is a school of thought that undecomposed wood chips are not a good thing to have buried in the soil around plants for other reasons). But if these chips you have been offered are in a separate pile and not mixed with the sand you could certainly use them as a mulch rather than an amendment.