This was an odd term which was introduced to modern paganism from the Theosophists and Swedenborg.
It is considered to be the pinnacle of human spiritual achievement in the afterlife; that is, it is the highest level, or ‘sphere’, of the afterlife we can hope to enter.
Leadbeater suggested Summerland was a place where a soul of good standing or moral action could await their next incarnation.
It can appear as singular or plural and with or without the definite article “the”: Summerland, The Summerland or The Summerlands.