In contemporary Jewish parlance, derech eretz, (literally "the way of the land") means something like good manners or the done thing. If there's no other firm reason for disapproving of something, you might say, "It's just not derech eretz."
The traditional meanings of derech eretz encompass, but are also much broader than, this. Derech eretz covers the basic norms of decent human behaviour which the Torah teaches, but which also in a sense precede Torah in forming the human personality: hence the maxim, "Derech eretz comes before Torah."
The requirements of derech eretz include speaking calmly and gently to people; eating sitting down like a human being, rather than while walking in the street, like an animal; dressing in clothes that are clean and presentable, but not gaudy or extravagant; walking naturally rather than pretentiously or affectedly, and generally behaving like a mensch.
In the famous passage from Pirkei Avot (Ethics of the Fathers, 2:2), "Torah is good, along with derech eretz," the phrase has a particular, specific meaning. Maimonides and the majority of commentators say it refers to doing productive work.
In the 19th century, "derech eretz" was given a new twist by Rabbi Samson Raphael Hirsch (1808-88), the German founder of modern or neo-Orthodoxy. He made "Torah with derech eretz" the rallying cry of his movement and understood it to mean the synthesis of Torah with all that's best in the surrounding culture.