From Homestar Runner Wiki
Tube Socks are the type of socks Strong Bad prefers. "Homely Boy Brand Huge Tube Socks", an imaginary company from modeling, is well endorsed by Strong Bad. According to Decemberween Short Shorts, he apparently gets at least one new pair of tube socks every Decemberween, and is seen writing a secret letter to them warning them that they might receive negative comments and/or negative burnings from him when he opens them up on Decemberween morning. Strong Bad wants to keep his love for tube socks a secret to save his reputation. The aforementioned burnings make it unknown whether or not Strong Bad actually wears the socks he receives.
Strong Sad prefers argyle socks.
Notably, there are times, such as in the sketch in the conclusion of Strong Bad's secret letter, where the term "tube socks" is used to describe socks with a discernible heel.