These concepts were hopefully covered in ILA in Semester 1 of Year 1 but you should read this Section 1.4 of Epp as graph theory is very important in many applications.
You will also find similar material in Levin Section 4.1.
In Inf2-IADS you will cover the type of graph called trees, which are connected but have no cycles. This is covered in Section 4.2 of Levin.