Modal verbs are a special kind of verbs that express how you “relate” to another verb. Specifically, they express capacity, possibility, obligation an desire.
In many European language, they share that they have a set of grammar rules that is specific to them.
In English for instance, modal verbs do not have “to” for their infinitive (there is no “to can”).
The definition of modal verb as well as the verbs themselves vary from language to language. German modal verbs do not exactly line up with English ones.