primary, secundary, tertiary en quaternary

The structure below shows 'methyl butane' at the left and 'dimethyl propane' at the right side.
Cabon atoms that are only connected to one other C-atom are called: primary C-atoms/ they always are situated at the end of a carbon chain.
C-atoms in between two others, we call secondary C-atoms.
Thus we also have tertiary C-atoms, connected to three others, and even quaternary ones, surrounded by four other C-atoms.

The words primary etc. are not exclusively used for the C-atoms. Also other atoms or atom groups can have this qualification.
For example, if one OH-group is connected to a secondary C-atom, we speak of a secondary alcohol, or a secondary hydroxy group.
In proteins you also will find those concepts primary etc. but then they mean something different.