Plans show where a thing goes. Details show how to build it. A detail marker on a plan gives the detail number and the sheet it is on.
Example: a detail shows how a wall pad attaches to a block wall. It gives the furring, the fasteners and the height above the floor.
Follow each detail marker in your area of work. Details hide costs that plans do not show, such as blocking, backing, sealant and trim. Check which trade the detail assigns each part to. If the detail and the spec disagree, send an RFI before the bid.