Blueprints are similar to recipes (called Engrams) learned through spending engram points, except you must have the blueprint with you in order to be able to craft the featured item. They are very useful in not having to spend engram points to learn to make the item, and instead simply using the blueprint. However they can not be used to unlock the next material tier. For example, to get chitin armour you still have to have the hide armour engrams, not the blueprints.
Blueprints can be found at Beacons or found as loot in caves. You can properly stock blueprints on a Bookshelf.
Blueprints can allow you to craft items that are better than their engram counterparts. Using a blueprint doesn't consume the blueprint itself in the process so you can use the same blueprint multiple times.
Blueprints often also give bonuses to the item crafted. e.g. cloth armor might have 11 armor and slightly increased insulation when crafted from a blueprint. This will be noted on the blueprint itself via its tooltip.
There is an increased cost in materials to craft higher tier items. an Apprentice Fabricated Pistol requires approximately 291% more materials on top of the usual materials. (137 Polymer, 78 Metal Ingots, and 117 Cementing Paste to the usual 35, 20, 30)
Blueprints have been found in four tiers so far: Primitive, Ramshackle, Apprentice, and Journeyman. Each tier blueprint boasts better stats than the last.