Ley lines are hypothetical alignments of archaeological sites, mountains, and other things on Earth's surface. There are some alignments that no one doubts. The Egyptian Great Pyramids are made to align with Orion's Belt and many stonehenges and woodhenges acted as calendar systems with each other and the stars.
While some consider these ley lines, when the controversial word is used another meaning is implied. Ley lines are usually brought up when some hidden knowledge, ancient culture, or massive geoengineering is allegedly involved. Today many New Age "geographers" attribute ley lines to magic discovered or made by ancient civilizations.
Part of the reason for ley lines being seen is that there is scared geography. Patterns exist in holy buildings: crosses for churches, east facing buildings for mosques, etc. Humans seek similiar patterns in nature as proof of their beliefs.
A key example of ley lines is the geography of Rennes le Chateau. The history of a priest selling fake masses to become rich turned into a treasure story which turned into a Jesus had a baby which turned into ancients built a geography/geometrical wonderland which turned into aliens etc. The geographical element revolves around several churches on mountains making a pentagram/star pattern. Do these ley lines prove some ancient force that gave a simple priest massive wealth? No, its all random (great read by the way).
Alignments in archeaolgy reveal complexity in civliziations and can help decode its culture. However, geographers must be careful not to fall in the ley line trap. Crazy ideas like Rennes le Chateau aliens hurt real research in alignments.