AIDA is an acronym used to illustrate the different phases of an advertising persuasion model.
AIDA stands for:
(to attract) Attention
(to spark) Interest
(to arouse) Desire
(to urge to) Action
AIDA model has been used for a long time in "traditional" marketing. The AIDA model applies to online advertising but can also be used for other digital marketing practices (email, landing page, etc.).
For an email, the attention and the interest are usually sparked by the subject line and the beginning of the message. Then the desire is aroused by the commercial sales argumentation while the urge to action is done within the message and on the landing pages.