The Golgi apparatus is an cell organelle, and it’s function can be best described as an hybrid between an assembly line and a logistics center. It sits close to the nucleus, and is needed to modify proteins into glycoproteins (proteins with different sugars attached) and then to transport these to the cell surface for integration into the cell membrane.

It consists of a stack of chambers made out of lipid membranes, and by the magic of having a different pH and different enzymes in the chambers, can selectively attach a “tree” of different sugars onto proteins. One important thing everyone has heard of that gets constructed here is the blood type! It depends on if only a basic sugar structure (0), or one or both of the attachments (A, B, AB) are attached onto surface proteins.

Something else that gets done here is the creation of “location tags”, which are used as a shipping label and to make sure that proteins are brought to the correct destination in the cell; you want to make sure your proton pumps for integration in lysosomes (which need a very low pH to help with degradation of all the stuff the cell doesn’t wanna deal with) really land at lysosomes and not for example the cell membrane.

It also provides the “transport vehicles”, the so called vesicles. They are created by budding off of the Golgi membrane, creating little lipid bubbles which contain the membrane-bound cargo and the shipping labels. These vesicles are also what is used for cell secretion, simply by merging with the cell wall and emptying their contents to the exterior when merging.

This is only a short overview - there’s a lot more going on than i want to put in a comment here. As always, https://en.wikipedia.org/wiki/Golgi_apparatus is a good starting point; i learnt most of what i know about this from Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter - Molecular Biology of the Cell