A conjunction is a word used to join together clauses within a sentence. In German there are two types of conjunctions, Coordinating conjunctions and Subordinating conjunctions.
Subordinating conjunctions are the trickiest so let's start with those. Subordinating conjunctions require the conjugated verb in the second clause to be moved to the end of the clause.
For example:
Ich denke, dass Deutsch eine schwierige Sprache ist = I think that German is a difficult language.
Coordinating conjunctions allow the verb to be placed in the same place as in the preceeding clause.
For example: Ich schwimme gern und ich gehe oft ins Schwimmbad = I like swimming and I often go to the swimming pool.
For a full list of Coordinating and Subordinating conjunctions please see the following link: https://www.lsa.umich.edu/german/hmr/Grammatik/Konjunktionen/Konjunktionen.html