He went into town every so often to buy supplies.

every 意思是"每个"，在这里与 so often 组成短语表示频率。 so 在这里是副词，表示程度，与 often 搭配使用。 often 意思是"经常"，表示频率。 every so often 是一个固定短语，意思是"偶尔、时不时地"，表示不频繁但会重复发生的动作。