Kadomatsu is a traditional decoration which is set out during New Years (about one week before and after Jan.1). The design/arrangement varies, with some variations being unique to specific regions. Generally, the kadomatsu is made of several of mature bamboo shoots, branches of pine needles and often some pine branches.
The bamboo and pine are the most basic and universal elements. Shoots of bamboo are hollow, so they represent purity or honesty. Also because they are hollow, they serve as a temporary home for spirits that visit their families for the holiday. They are usually displayed in pairs on either side of doorways, thus the name kadomatsu, 門松, or gate-pines. The matched pair represent male and female (I guess the spirits have segregated facilities like a bathhouse?)
In Japan New Years is a time for family gathering while Christmas Eve is the time for romance (reverse to American tradition).
The pine elements represents the persistently blooming life of that evergreen tree during the winter season.
The plum tree represents prosperity because the plum tree is the first to bloom, even while the weather outside remains cold.
These three plants together are thought of as the “three friends of winter” (“suihan sanyou”) and placed together in context are known as “shou-chiku-bai” (松竹梅). “Shochikubai” is also the name of a popular brand of sake.
This is confusing because the normal pronunciation of these character is different: pine = matsu 松, bamboo = take 竹 and plum = ume 梅. You can also see these secondary pronunciations in some other words: for example the word for a “plum grove” is 梅林, which is the word for plum (ume) and forest (hayashi) together that make the word “bai-rin.”
The trio of plum-bamboo-pine also appears in Japanese culture as a traditional version of gold-silver-bronze. You can see this method used in rating grades of goods; plum=cheap, bamboo = medium level, pine = highest grade. For example, some restaurants use this scale for meal sets and kendo goods sell grades of equipment (especially himo) in these terms.