Covalent Bonding

[Aerlinn]

With diagrams like these which show atoms covalently bonded, what do the dotted lines and the triangle shape represent? I know they help in representing the 3 dimentional structure of the molecule, but as to what they exactly mean, can anyone tell me? (Because when I look at it, I cant see a 3D shape since I dont know what the dots/ triangles represent)