"Pollarding trees" means cutting them back nearly to the trunk, so as to produce a dense mass of branches. It is sometimes done today for aesthetic purposes and/or to keep a beloved tree from outgrowing its bounds, necessitating removal. But traditionally, it was done for other reasons: the cut branches were either fed to livestock (fodder), burned as fuel or used to make things.
Pollarding begins on young trees, and the process is repeated throughout the life of the tree. Only certain types of trees are suited to pollarding.
Now that you know what "pollarding" means, compare it to "topping."