'Brookside' means 'along the brook'. Brookside flowers grow next to a brook. A brookside walk is a walk or pathway

alongside a brook.

It refers to the stream. A brook is a small stream, so brookside means along the stream.

Thank you, I've got it.

Thank you for your remarks, but I translate the guide from Essex to russian. So I can understand the word "brookside". At the first glance, it is a stream, but I have found this word in the names of flowers: brookside flamenco, brookside annabella, brookside rose, etc.This flower is  pelargonium or geranium. Could you give me definition of this word in English?

You are so lucky of visiting this area.

Half an hour's drive south, near Great Dunmow, and you'll find the beautifully restored garden of Easton Lodge, which were originally laid out by Harold Peto in 1902, while another half an hour takes/brings you down to the Gibberd Garden, just outside Harlow.
Created by Sir Frederick Gibberd, planner of Harlow New Town, it is a fascinating plot, filled with sculpture, ceramic pots, and architectural salvage.  It boasts formal lawns and flower beds, a brookside walk with a waterfall, a wild garden, a gazebo with a formal pond, an arboretum, an island fort with a drawbridge... and even more besides!

As Susan612 has said, 'brookside' refers to a path alongside a stream or 'brook'.  You could say  'a walk alongside the stream, with a waterfall'.

I have walked in this area, but have not seen the gardens of Easton Lodge - clearly worth a visit.