What is the meaning of 及 in the compound word 危及?
及 means to reach. Some examples:

及格: to pass a test. Literally 'to reach the bar'.
长发及腰: 'long hair reaching waistline'.
涉及: to involve. Literally 'intervention reaches to'.
提及: to mention. Literally '(the scope of) mentioning reaches to'.
力所能及: within grasp. Literally 'capability that (one) can reach'.