There are many Iyashikei series but the term itself seems to be misunderstood as a genre or sub genre of manga and anime. This is not the case, but first lets clear up any misunderstand of the term isyashikei, Iyashi means healing, and the term kei refers to type. So iyashikei means something has healing faculties. However, what does that exactly mean?
The healing does not refer to physical or physiological sense instead it is a form of mood regulation or in simpler terms; it is a form of relaxation. The origin of this idea does not originally come from manga or anime instead it comes from a wider movement in Japan that eventually reached animation and comics.
The trend for Iyashikei reached a boom in Japan the late 1990s, it was aided by the difficult period that Japan faced during that period of the 90s as it suffered tragedies like the Kobe earthquake in 1995 but also the Aum Shinrikyo terrorist attack in the Tokyo subways in 1997. In addition to all that, there was the stagnation of the Japanese economy after the bubble economy collapsed all within that same decade. It was a completely different period from the economic miracle of the 60s that lead to a growth optimism that partially continued in the 70s too.
These events left a scathing mark in Japanese society which faced economic restructuring which continues to this day with the persistence of slow economic growth. It was in this context that there was a development of a boom of 'iyashi' or healing. This came in the form of a interest in pursuing things like creams, therapy but also music and literature that could provide a form of relaxation from the stress of modern life. These all aimed to provide a soothing experience in music for example through ambience calming music and in literature through stories that bring optimism and other positive feelings. It is precisely through this process of soothing the mood of the recipient and bringing out a positive feeling that it is a 'healing' experience.
Of course, this trend spilled over in to the visual medium that is animation and many anime try to replicate this healing effect. This is possible because it can simply embed the iyashikei aspects of literature and music and blend them alongside the animation to attempt to provide an even greater healing experience.
Anime fits well for Iyashikei for various aspects but one of them is the usage of music which as highlighted above was also a key part of the healing boom. Healing music is innately tied to natural sounds like rainfall, oceans waves or the cries of animals like whales accompanying low sounding instrumental tracks. These sounds all fit in the healing idea because they have a soothing element by posing a closer to nature link away from the hustle and bustle of the city.
Iyashikei anime excel at using these and visual imagery to visualise a peaceful place as it drifts of into scenery animation with the ambient nature sounds for a occasional moments. This together with a more down to earth ambient quite a more natural feel and regulates the mood by emulating relaxing sounds of nature which in turn makes fits the countryside setting of the series. The visual aspect which is the most important in animation mixes well exactly because of this blend of the music and scenery which is a key part of the healing series.