Just like the kanji characters express, 季語 means “season word”, i.e. the words or phrases used to express the season.
季語 are very important words in Japanese poetry - certain types of poetry, such as haiku, always have a 季語 type reference included. 季語 is also often used in formal letters, such as wedding invitations. However, recent generations prefer a less formal style and omit 季語 words since the internet has made communication much easier and more frequent, but the tradition still remains for the older folks!
Some examples of 季語 could be 雪, or “snow,” used to represent the for winter time, and 桜, or “cherry blosson,” as a 季語 for spring time (more specifically, late March to early April, when cherry blossoms bloom). 季語 isn’t just restricted to things in nature, but events or man made objects can also be used as 季語. For example, スキー, or “skiing” is a 季語 for winter, and ビヤガーデン, or “beer garden” is a 季語 for summer since outside beer halls only usually open during the summer.
There are many types of 季語 out there - if you are interested in Japanese poetry or just how the Japanese view nature, I recommend to check them out!
In this letter, the seasonal words are used improperly.
We must add seasonal words in a haiku.