I would imagine its just compost or good quality soil placed around the tree in order to guard it from, like you say, strimmers and such. For total weed control, you could either line the area with a weed guard or place bark chips above it. Although if you didnt mind tackling small arounts of weeds, it wouldnt really be a problem.
For pruning, start with the older branchs and cut them well back. Prune the higher branches to be shorter than the lower, and make sure your shears are sharp!
Cut diseased and dead branches too, just don't go too mad cutting back as you could kill the tree.